London's Ultimate Paid Ads Agency Vetting Guide

I see this all the time. A founder in London, sharp as a tack, has built a brilliant product but is getting absolutely nowhere with paid ads. They know they need help, so they start looking for an agency. And that's where the real trouble begins. They're drowning in a sea of agencies all claiming to be "data-driven," "award-winning," and based in some trendy co-working space near Old Street. It's a proper nightmare trying to tell the good from the bad, and most people make the wrong choice.

The problem is that most founders are looking at the wrong things. They get impressed by a slick sales deck, a long list of client logos (that often includes work they did ten years ago), or a promise of "guaranteed results" that's not worth the paper it's written on. You end up burning through thousands of pounds, wasting months of time, and concluding that "paid ads just don't work for my business." That’s rubbish. What didn't work was your process for finding a partner who actually knows what they're doing.

This is a no-nonsense guide to cutting through the waffle and finding a paid ads agency in London that will actually make you money. It's not about finding the cheapest, it's about finding the most profitable. Let's get started.

So, why is it so hard to pick a good agency?

Before we get into the "how," we need to talk about the "why." Why do so many smart founders end up signing contracts with agencies that are, frankly, not up to the job? It usually comes down to a few common myths and misconceptions about what a "good" agency looks like. It's easy to see how the difficulty differentiating between PPC agencies can lead you down the wrong path.

The first trap is the obsession with image. A swanky office in Canary Wharf or a team photo that looks like a casting call for a tech startup advert tells you absolutely nothing about their ability to manage a multi-platform ad campaign for a complex B2B SaaS product. It just tells you they're good at spending money, which might well be yours. I've seen one-man bands operating from their spare room run rings around big agencies because they have deep, specific expertise.

The second trap is the "big brand" fallacy. Seeing a huge household name on an agency's client list feels reassuring, right? But what does it actually mean for your business? Often, that big brand has a multi-million-pound budget and an entire in-house team collaborating with the agency. The strategy that works for them is almost certainly not the right one for a startup trying to get traction. You need an agency that lives and breathes your world – the world of tight budgets, rapid testing, and the desperate need for a measurable return on every single pound spent. This is especially true if you are a B2B business with a high-touch sales process.

And the biggest trap of all? The promise. Any agency that "guarantees" you a specific ROAS or a number one ranking on Google is either lying or naive. Tbh in paid advertising, you can't really promise anything. There are far too many variables – market conditions, competitor actions, platform algorithm changes. A real expert will never promise a specific result. Instead, they'll talk about their process, their methodology for testing, and show you case studies where they've achieved brilliant results for businesses like yours. They sell proof, not promises.

How do you spot genuine expertise?

Alright, so we know what to avoid. How do we find the real deal? It starts and ends with evidence. You need to become a detective, and their case studies are your crime scene. This is the single most important part of the vetting process.

Don't just skim them. Interrogate them. Here’s what you're looking for:

-> Relevance is everything. If you're a FinTech founder, you need to see case studies for other FinTech or at least B2B SaaS companies. An agency that gets amazing results for an ecommerce fashion brand is probably not the right fit. Their experience is in a completely different universe. I remember one campaign we ran for a B2B software client generated 4,622 registrations at just $2.38 each on Meta Ads. Another for a medical job matching SaaS reduced their Cost Per User Acquisition from a painful £100 down to just £7. That's the kind of specific, relevant result you want to see.

-> Look for the numbers. Vague statements like "increased brand awareness" or "drove significant traffic" are red flags. You need hard metrics. How much was the ad spend? What was the revenue generated? What was the ROAS (Return On Ad Spend)? What was the CPL (Cost Per Lead)? If the numbers aren't there, they're either hiding something or they don't track them properly, which is even worse.

-> Understand the "how." A good case study doesn't just show the result; it gives you a glimpse into the strategy. Did they use LinkedIn Ads to target specific job titles? Did they use Meta's lookalike audiences based on existing customer data? Did they use Google Search ads to capture high-intent traffic? It should sound like they solved a difficult puzzle, not just got lucky. It should give you a taste of the expertise you'll be paying for.

Here's a simple flowchart to help you cut through the noise when you're reviewing an agency's work. If they don't make it through this process, they're not worth your time.

What should you actually be paying for an agency in London?

This is the million-dollar—or rather, thousand-pound—question. Pricing in London can feel all over the place, but it generally falls into a few buckets. Understanding these is the first step to figuring out if you're getting a fair deal. Knowing the landscape is crucial when trying to understand London ad agency pricing.

1. Monthly Retainer: This is the most common model. You pay a fixed fee each month for the agency to manage your campaigns. This can range from £1,500 for a smaller freelancer or boutique agency to £10,000+ for a larger, more established firm. The fee should reflect the complexity of your campaigns and the level of strategic input you're getting.

2. Percentage of Ad Spend: Here, the agency takes a cut of what you spend on ads, typically between 10-20%. This model can work well as it incentivises the agency to help you scale, but you need to be careful. An unscrupulous agency might encourage you to increase spend even if it's not profitable, just to boost their fee.

3. Performance-Based: This sounds like the dream ticket – you only pay for results, like a fee per lead or a percentage of sales. However, it's quite rare for pure-play paid ads management because so many factors outside the agency's control (your website conversion rate, your sales team's effectiveness, your pricing) affect the final outcome. It's more common in affiliate marketing.

So what's right for you? It depends. For most startups and SMEs, a fixed monthly retainer makes the most sense as it's predictable. But the amount has to be right. Paying a £5,000 retainer to manage a £2,000 ad spend is madness. A good rule of thumb is that the management fee should be a sensible fraction of your total ad budget.

To give you a better idea, here’s a calculator to estimate what you might expect to pay based on your ad spend. It’s not gospel, but it'll ground you in reality.

How to know if they can actually think strategically

A good agency doesn't just push buttons in the ad platforms. They are strategic partners. They should be obsessed with your customer. If they start the conversation by asking about your target keywords or ad budget, they're a tactician. If they start by asking about your customer's biggest pain points, they're a strategist. There's a huge difference.

You need to forget the sterile, demographic-based profile your last marketing hire made. "Companies in the finance sector in London with 50-200 employees" tells you nothing of value. A top agency knows this. They know your Ideal Customer Profile (ICP) isn't a demographic; it's a nightmare. It's the specific, urgent, expensive problem that keeps them awake at night.

For a legal tech SaaS, the nightmare isn't 'needing document management'; it's 'a partner at a magic circle firm missing a critical filing deadline and exposing the firm to a malpractice suit.' For a fractional CFO service targeting startups, the nightmare isn't 'doing the books'; it's 'running out of cash before the next funding round and having to lay off the team'.

On your initial call with a potential agency, turn the tables. Ask them: "How would you go about defining our ideal customer?"

A bad answer will be about demographics, interests on Facebook, or job titles on LinkedIn. A good answer will be about pain points, problems, and 'nightmare scenarios'. They'll ask you about your best customers, what triggered them to buy, and what language they use to describe their problems. This is how you seperate the button-pushers from the business-builders. It's a critical part of the vetting framework for a B2B ad agency in London.

What's the one number that truly matters?

Most conversations about paid ads get stuck on metrics like Cost Per Click (CPC) or Cost Per Lead (CPL). While these are important, they don't tell the whole story. The real question isn't "How low can my CPL go?" but "How high a CPL can I afford to acquire a truly great customer?" The answer lies in its counterpart: Lifetime Value (LTV).

An agency that doesn't ask about your LTV, or can't help you calculate it, is flying blind. They're optimising for cheap leads, not profitable growth. Here's the basic maths:

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

Let's say you run a SaaS business. Your average customer pays you £300/month (ARPA), your gross margin is 80%, and you lose 5% of your customers each month (churn).
LTV = (£300 * 0.80) / 0.05 = £240 / 0.05 = £4,800.

Each customer is worth £4,800 in gross margin to you. A healthy LTV:CAC (Customer Acquisition Cost) ratio is at least 3:1. This means you can afford to spend up to £1,600 to acquire a single customer. If your sales team converts 1 in 5 qualified leads, you can afford to pay up to £320 for a single qualified lead. Suddenly, that £150 lead from LinkedIn Ads doesn't look so expensive anymore, does it? It looks like a bargain. This is the maths that unlocks aggressive, intelligent growth.

Use this calculator to get a handle on your own numbers. It will change the way you think about your ad spend.

How to nail the first conversation

You've done your research, shortlisted a few agencies based on their case studies, and now it's time to get on a call. This is not a sales pitch. This is an interview, and you are the one hiring. Your job is to extract as much information as possible and get a feel for whether they are genuinely sharp, or just good at talking.

The biggest mistake founders make is they let the agency lead the entire conversation. You need to come prepared with questions that cut to the chase. Here are a few to get you started:

• "Walk me through a campaign you ran for a client similar to us that didn't work initially. What went wrong, and what did you do to fix it?" - This tests for honesty and problem-solving skills. Anyone who says all their campaigns are winners from day one is lying.
• "What's your process for creative testing? How do you decide on messaging and ad formats?" - You're looking for a structured approach, not just "we'll try a few things and see what sticks". They should talk about audience pain points, A/B testing, and iterating on winning concepts.
• "How will you communicate performance to us? Can I see an example of a report?" - You want clarity and transparency. A good report focuses on the metrics that matter (CPL, CPA, ROAS) and provides insights, not just a data dump.
• "Who will actually be working on my account? Can I speak to them?" - Often, the senior person who sells you the dream won't be the one in the trenches. You need to know who is managing your money day-to-day and be confident in their ability. This is particularly important when weighing up a SaaS marketing consultant or a larger agency.

Finally, and most importantly, look for an offer of value. The best agencies won't just try to sign you up on the first call. They'll offer to do a free, no-obligation audit of your existing ad account or a deep-dive strategy session. We do this for all potential clients. It's a chance for us to show our expertise and for the client to get a huge amount of value upfront. It builds trust and demonstrates capability far better than any sales deck ever could. An agency that's confident in its ability isn't afraid to give away some of its expertise for free.

The final decision

Making the final choice is tough, but by following this process, you'll be deciding between a few genuinely strong contenders rather than taking a punt on a random agency from a Google search. To help you make that final call, I've put my main recommendations into a table below. Think of it as your final checklist before you sign on the dotted line.

Choosing an agency is a huge decision. It can be the difference between stagnating and achieving explosive growth. While this framework will give you a massive advantage, the process can still be daunting and time-consuming. Sometimes, bringing in an expert pair of hands is the fastest way to get results.

If you've read this far and you're thinking that your business could benefit from a team that lives and breathes this stuff every day, we should probably have a chat. We offer a completely free, no-strings-attached 20-minute strategy session where we'll look at your business, your goals, and give you actionable advice you can use immediately, whether you decide to work with us or not. If you're serious about growing your business with paid ads, schedule your free consultation today.