London's Paid Ad Agencies: Founder's Vetting Guide

Trying to find a decent paid ads agency in London can feel like a nightmare. You're drowning in a sea of agencies, all based around Old Street or in Soho, all with slick websites, and all promising to '10x your growth' with 'data-driven strategies'. It’s all nonsense, and it’s designed to be overwhelming. The truth is, most of them are selling the same generic service with different branding.

The problem is that founders and marketing managers don't have a proper framework for cutting through the noise. They get swayed by a confident sales pitch or a fancy office, and end up signing a 6-month contract with a team that has no real experience in their specific market. This guide is the framework you're missing. It's a no-nonsense, brutally honest look at how to actually vet and choose an agency in London that won't just burn your cash. It's not about finding the cheapest option; it's about finding a genuine partner who can deliver a return.

Why are all London agencies saying the same thing?

The first thing you have to understand is that the paid media agency model, especially in a hyper-competitive market like London, is built on sales. They hire smooth-talking sales reps who know all the right buzzwords – 'synergy', 'optimisation', 'omnichannel approach'. They'll show you glossy charts that go up and to the right, but they rarely represent a business like yours.

Any agency that 'guarantees' results is either lying or naive. Paid advertising is volatile. A campaign that works today might stop working tomorrow because a competitor just raised a funding round and has doubled their ad spend, or Meta decided to change its algorithm again. A real expert knows this. They don't promise certainty; they promise a rigorous, intelligent process of testing and iteration to find what works. Promises are a red flag. A process is a green flag.

They’ll talk about impressions, clicks, and click-through rates (CTR). These are vanity metrics. They are easy to manipulate and mean absolutely nothing for your bottom line. I can get you a million impressions for a few hundred quid by showing your ads to people in countries with low-quality traffic, but it won't generate a single sale. You need an agency that talks about business metrics: Cost Per Acquisition (CPA), Return On Ad Spend (ROAS), and Lifetime Value (LTV). If they don't bring these up on the first call, they're not the right fit. This is particularly true for any serious business; you need a partner who understands how to make paid advertising profitable, not just busy. We've seen so many accounts where a previous agency has focused on getting cheap clicks, but the traffic just doesn't convert. It's a complete waste of money. To learn how to spot and fix this, you should check our detailed guide on how to fix Facebook ads that are not converting despite good traffic.

So, how do I know if they can actually help my business?

You ignore 90% of their website and go straight to their case studies. This is the only part of their marketing that actually matters. But you need to know how to read them. A vague testimonial saying "They were great to work with!" is useless. You are looking for proof, not platitudes.

Here’s what to look for:

-> Industry Relevance: Have they worked with a business like yours before? Not just 'B2B', but 'B2B SaaS for the legal tech sector'. Not just 'eCommerce', but 'direct-to-consumer subscription boxes for the health market'. The more specific, the better. An agency that's scaled a women's apparel brand to a 691% return on Meta ads has a completely different skillset to one that gets B2B leads on LinkedIn for $22 CPL. The strategies don't translate.

-> Real Metrics (£): The results must be in pounds. This shows they have genuine experience in the UK market, which has its own unique consumer behaviours and costs. It also needs to show business outcomes. For instance, one of our clients, a medical job matching SaaS, had a Cost Per User Acquisition (CPA) of over £100. We managed to reduce it to just £7. A good case study would not just state that result; it would provide key details, such as the platforms used—in this case, a combination of Google Ads and Meta Ads. The 'how' is almost as important as the 'what'.

-> Honesty About Challenges: The best case studies aren't just a highlight reel. They talk about the initial problems, the failed tests, and the strategic pivots. This shows they are thinkers, not just button-pushers. It shows they understand that advertising is a process of discovery, not a magic bullet.

If an agency can't show you a detailed case study from a business similar to yours, with real financial results, they are asking you to pay for their education. Don't do it.

What should I ask them on the first call?

You need to flip the script. The discovery call is not for them to sell to you. It's for you to interview them. A good agency will welcome this; a weak one will be thrown off balance. Their goal is to qualify you as a client. Your goal is to qualify them as a competent partner. Tbh if someone asks us for references after we've shown them detailed case studies and given them a free account audit, it's an instant sign we're not a good fit because the trust just isn't there.

Here are some questions that cut through the usual sales patter:

1. "Talk me through a campaign for a client similar to me that failed. What went wrong and what did you learn?"
This is the most important question you can ask. It tests for honesty, humility, and critical thinking. Every single agency has had campaigns fail. The ones who pretend they haven't are liars. The good ones see failure as a data point, an opportunity to learn and improve. Their answer will tell you everything about their culture and process.

2. "Based on what you know about my business, what would be the first two or three things you'd test in our ad account?"
This tests their strategic thinking on the spot. A generic answer like "We'd start with an audience analysis and then build a full-funnel strategy" is a cop-out. A great answer would be specific: "Given you're a B2B SaaS, I'd immediately want to test a LinkedIn campaign targeting job titles like 'Head of Sales' at companies with 50-200 employees, running against a Google Search campaign for high-intent keywords like 'sales CRM for small business'. I'd want to see which channel drives a better quality of demo request." This shows they are already thinking about your specific problem.

3. "Who, specifically, will be working on my account day-to-day? What's their experience level?"
This is to avoid the classic 'bait and switch', where you're sold by the charismatic agency director but your account is handed off to an overworked junior account manager who is fresh out of university. You want to know who is actually in the trenches, managing your budget. A good agency will be transparent about this and might even bring the account manager onto a second call. If you're looking for deep expertise, you might find that vetting a specific consultant rather than a large agency is a better path.

A good agency should leave you feeling like you just got a free consultation. You should come away from the call with 2-3 actionable ideas you could implement yourself. We do this all the time; our free strategy review is designed to provide genuine value upfront. If an agency just talks about themselves and their process for an hour, they're not the one.

How much is this actually going to cost me?

This is where most founders get it wrong. They shop for an agency based on the monthly retainer fee, trying to find the cheapest option. This is a false economy. A cheap agency that delivers no results is infinitely more expensive than a pricier one that delivers a 5x return.

In London, you'll see a few common pricing models:

• Percentage of Ad Spend: Usually 10-20%. This can be okay, but it can also incentivise the agency to simply get you to spend more, rather than spending more efficiently.
• Flat Retainer: Common in London, anywhere from £2,000 to £10,000+ per month. This is predictable, but you need to be sure they are motivated to perform and not just coast along.
• Performance-Based: Very rare and often has complex terms. Be wary of this unless the goals are crystal clear (e.g., pay per qualified lead).

The agency fee is irrelevant without understanding your business's own numbers. The real question isn't what the agency costs, but "How much can I afford to pay to acquire a customer?" To answer that, you need to calculate your Customer Lifetime Value (LTV). Most businesses I speak to haven't done this properly. It's the most powerful number you can know.

Here's how you work it out:

LTV = (Average Revenue Per Customer Per Month * Gross Margin %) / Monthly Customer Churn Rate

Let's say you run a SaaS company. Your average customer pays you £500/month, your gross margin is 80%, and you lose 4% of your customers each month (churn).
LTV = (£500 * 0.80) / 0.04
LTV = £400 / 0.04 = £10,000

Each customer is worth £10,000 in gross margin to you. A healthy business model aims for a 3:1 LTV to Customer Acquisition Cost (CAC) ratio. This means you can afford to spend up to £3,333 to acquire a single new customer. Suddenly, paying an agency £3,000 a month to bring you two or three new customers seems like a fantastic deal, doesn't it? Knowing your LTV completely changes the conversation from "How much do you cost?" to "Can you acquire customers for me under £3,333?" It's a much more intelligent way to approach your marketing budget, and you can find a more detailed breakdown in our guide to London ad agency pricing.

Does it matter if they're a B2B specialist?

For a business in London, yes, it matters immensely. London's economy is heavily weighted towards B2B sectors like finance, technology, legal services, and consulting. Advertising to a CFO in Canary Wharf is a completely different universe to advertising a new fashion brand on Instagram. If an agency doesn't understand this distinction, they will incinerate your budget.

B2B advertising is defined by:

• Longer Sales Cycles: A purchase decision isn't made in 5 minutes. It can take months and involve multiple stakeholders. The ads are not designed for an impulse buy; they are designed to start a conversation or generate a qualified lead for your sales team.
• Different Platforms: While Meta can work for some B2B, LinkedIn is often the primary channel. It allows for incredibly specific targeting based on job title, company size, and industry. A B2B specialist will have deep expertise in LinkedIn Ads, which is a far more complex and expensive platform than Meta.
• Different Offers: You're not selling a product; you're selling a solution to a complex business problem. The call to action isn't "Buy Now". It's "Request a Demo", "Download the Whitepaper", or "Book a Consultation". The entire funnel is different.

A generalist agency will try to apply a B2C mindset to your B2B campaign and it will fail. They'll focus on clicks instead of lead quality. They'll be shocked at the high CPLs on LinkedIn (£20-£100+ is normal) because they're used to £2 CPLs on Facebook. You need an agency that lives and breathes the B2B world. They'll understand the nuance and have case studies to prove it, like the campaign I mentioned earlier where we generated high-quality B2B leads for $22 each on LinkedIn. This is the kind of specific experience you should be looking for.

Shouldn't I just hire someone in-house?

It's a fair question, especially for a well-funded London startup. The desire to have someone fully dedicated to your brand is strong. But it's often a financially and strategically poor decision in the early stages.

Let's look at the real cost of an in-house hire in London. A decent paid media manager will command a salary of at least £50k-£60k. On top of that, you have National Insurance contributions, pension, equipment, software subscriptions (£££), and management overhead. Your true cost is closer to £75k-£90k per year. And for that, you get one person's brain and one set of experiences. If they leave, all that knowledge walks out the door.

Compare that to an agency. A typical retainer might be £3k-£5k per month (£36k-£60k per year). For that, you're not just getting one person. You're getting access to a whole team: a strategist, a copywriter, a designer, and an account manager. You're benefiting from their collective experience working across dozens of other accounts. They see what's working right now in different industries and can apply those learnings to your campaigns. That cross-pollination of ideas is something an in-house person can never replicate.

An agency offers more expertise for less money and lower risk in the beginning. Once you're spending £50k+ per month on ads, the economics might shift towards building an in-house team. But before that, an agency is almost always the smarter choice. There are many factors to consider, and our complete guide on agency vs in-house costs breaks it down in more detail.

So, what's the plan?

Choosing an agency is one of the most important marketing decisions you'll make. Getting it right can transform your growth trajectory. Getting it wrong can set you back six months and tens of thousands of pounds. The key isn't to find a magical agency with a secret formula. The key is to run a rigorous, disciplined vetting process based on proof, not promises.

You need to become an expert at buying agency services. Demand relevance, demand transparency, and demand strategic thinking from day one. Any agency worth their salt will respect and welcome this approach. The ones who try to evade your questions with jargon are the ones you need to avoid.

To make it simple, I've broken down the entire vetting process into a final checklist for you below. Use it as your guide when you're assessing potential partners.

Ultimately, the best way to know if an agency is the right fit is to get a taste of their expertise. This entire process is about reducing your risk before you commit. If you're currently running ads or thinking about starting, and you want a second opinion from a team of specialists who live and breathe this stuff, we offer a completely free, no-obligation strategy consultation. We'll look through your account, tell you honestly what we think is working and what isn't, and give you a clear plan of action. It's our way of proving our value before you ever pay us a penny.

Hope that helps!