London Ad Agencies: The Founder's Bulletproof Vetting Guide

Finding a good paid ads agency in London feels a bit like trying to find a seat on the Central Line at 8 AM. It's crowded, competitive, and you're likely to end up next to someone you'd rather avoid. The city is swimming in agencies, from massive networks in Canary Wharf to tiny two-person shops in Shoreditch, and most of them are, to be blunt, distinctly average. They're great at writing proposals and talking a good game, but fall flat when it comes to delivering actual, measurable results for your business.

The problem is that most founders approach hiring an agency all wrong. You're looking at the wrong signals, asking the wrong questions, and getting distracted by things that have zero impact on your bottom line. You see a slick website, a few impressive client logos, and you assume you've found 'the one'. A few months and several thousand pounds later, you're looking at a dashboard full of vanity metrics and wondering where it all went wrong. This guide is here to change that. I'm going to walk you through how to see past the noise and identify a genuine growth partner, not just another supplier.

So, why is your current method for finding an agency failing you?

Let's be honest. You've probably been told to look for an agency with a great portfolio, lots of five-star reviews, and a professional-looking team. This is terrible advice. It's like judging a restaurant by the font on its menu. A slick presentation tells you they've invested in a good designer, not that they can grow your business.

The truth is, the best agencies are often too busy delivering results for their clients to spend all their time polishing their own marketing. Their websites might be simple, even a bit dated. They aren't churning out endless content on LinkedIn. They're in the trenches, managing campaigns, analysing data, and making their clients money. Their reputation comes from word-of-mouth from happy clients, not from winning industry awards for 'Best Agency Website'.

You also get seduced by size and location. You visit a flashy office near London Bridge, get offered a fancy coffee, and think, "These guys must be successful." What you should be thinking is, "My retainer is paying for this expensive rent and that barista-quality coffee machine." The best talent doesn't need a postcode in EC1 to be effective. A small, focused team of true experts will almost always outperform a large, bloated agency where your account is handed off to a junior exec the moment you sign the contract. You need to stop looking for proxies for success and start looking for actual evidence of expertise. And if you're a B2B founder, finding a partner that truly gets the long sales cycles and complex funnels is even more difficult; you need a specialist, not a generalist who runs campaigns for corner shops one day and FinTechs the next. For more on this, check out our guide on how B2B companies in London should choose their agency partner.

How can you spot a genuinely good agency from a bad one?

A good agency doesn't lead with promises, they lead with questions. On your first call, if they're immediately talking about "smashing your KPIs" and "delivering 10x ROAS" without having asked a single deep question about your business model, your customers, or your margins, hang up. They're selling you a dream, not a strategy.

A real expert will want to understand the nuts and bolts of your business. They'll ask about your customer lifetime value (LTV), your current cost per acquisition (CPA), your sales cycle, and your profit margins. They'll sound more like a financial consultant than a marketer. Why? Because they know that paid advertising doesn't exist in a vacuum. It's a tool to drive profitable growth, and you can't do that without understanding the underlying business economics. They'll want to get under the bonnet and see how the engine works before they start trying to tune it.

This is the process a genuine expert follows, and it’s what you should be looking for.

The most powerful thing a good agency can offer you upfront is a free audit or strategy session where they actually look at your ad accounts. This isn't a sales pitch. It's a taster of their expertise. They should be able to look at your campaigns and, within 20-30 minutes, give you 2-3 concrete, actionable insights that you could implement yourself tomorrow. That's the ultimate proof of competence. If they're unwilling to do this, or want to charge you for it, they're probably not confident in their ability to provide value. They're hiding their expertise behind a paywall because it might not be that impressive.

How do you read their case studies without getting tricked?

Ah, the case study. The cornerstone of every agency's sales pitch. And most of them are complete fiction. Okay, maybe not fiction, but they are definately masterpieces of carefully selected data and omitted context. A case study that screams "We got 10 Million Views for a Luxury Brand!" is utterly meaningless. Who cares? Did those views lead to sales? What was the cost per view? What was the return on ad spend? It's a vanity metric designed to impress people who don't know better.

You need to learn to dissect a case study like a detective at a crime scene. Here’s what to look for:

• -> The Starting Point: Where was the client before the agency started? A good case study establishes a clear baseline. For example, one of our clients, a medical job matching SaaS, had a Cost Per User Acquisition of £100. Knowing this starting point gives context to the final result.
• -> The 'How': What was the actual strategy? "We used Meta Ads" is not a strategy. What targeting did they use? What creative angles did they test? Why did they choose that platform? A great case study explains the thinking. When we reduced that client's CPA from £100 down to just £7, it wasn't magic. It was a methodical process of testing new audience segments on Meta and restructuring their Google Ads campaigns to focus on high-intent keywords. That's the detail that matters.
• -> The Real Metrics: Ditch the vanity metrics. Look for the numbers that affect the bottom line: Return on Ad Spend (ROAS), Cost Per Lead (CPL), Customer Acquisition Cost (CAC), and Lifetime Value (LTV). If they can't provide these, they're either hiding something or they don't track what actually matters.
• -> The Timeframe: How long did it take to get these results? "We achieved 618% ROAS" sounds amazing, but if it took two years, it's less impressive. Look for results delivered in a reasonable timeframe. For instance, one campaign we ran for a client selling online courses generated $115k in revenue in just 1.5 months.

When you're on a call with a potential agency, don't just ask "Can I see a case study?". Ask them to walk you through a case study for a client similar to you. Interrupt them. Ask questions. "Why did you choose that audience?" "What was the click-through rate on that ad creative?" "How did you measure the impact on their overall revenue?". Their ability to answer these questions confidently and in detail will tell you if they were actually responsible for the results or if they're just reading from a marketing slick. For a complete checklist, you might find our founder's vetting framework for London agencies quite helpful.

What should you actually pay for a London agency?

Agency pricing in London is all over the place, and it can be completely opaque. Understanding the models is the first step to not getting ripped off. Generally, you'll encounter three main types:

1. Percentage of Ad Spend: This is common, usually 10-20%. The problem is it creates a potential conflict of interest. The agency is incentivised to make you spend more, not necessarily more efficiently. It can work, but you need to be sure they're focused on ROAS, not just burning through your cash.
2. Flat Monthly Retainer: This is often better. It's predictable, and the agency's incentive is to deliver great results to keep you as a client. The fee is based on the scope of work, not how much you spend. This is the model most good, results-focused agencies prefer.
3. Performance-Based: This sounds like the dream ticket – "you only pay for results!" – but it's often more complex. The agency takes a percentage of revenue or a fee per lead. The catch is that the percentages can be high, and it can lead to disagreements over attribution. It's rare for top-tier agencies to offer this for new clients because it puts all the risk on them, especially if your product or website isn't proven to convert.

So what's a fair price? It depends massively on your ad spend and the complexity of the work. A simple campaign for a local service might be a £1,500/month retainer. A multi-channel, international B2B SaaS campaign could easily be £5,000-£10,000/month or more. Don't cheap out. With paid ads, you absolutely get what you pay for. A cheap agency will cost you a fortune in wasted ad spend and missed opportunities. A great agency might have a higher retainer, but they'll generate a return that makes their fee look like a rounding error. I've built a calculator below to give you a rough idea of what you should expect to pay based on your ad spend.

Ultimately, the price needs to make sense within your business model. If you truly understand the potential ROI an agency can bring, the fee becomes an investment, not a cost. That's why the initial deep dive questions about your LTV and margins are so important.

Why asking for references is a massive mistake

This is probably the most controversial piece of advice I'll give you, but it's one of the most important. Do not ask a potential agency to speak to one of their current clients. A top-tier agency will see this as a huge red flag and will likely decline to work with you.

Think about it from their perspective. They have already provided you with detailed case studies. They have likely spent 30-60 minutes on a call with you, giving away free advice and auditing your account. They have been completely transparent. If, after all that, you still don't trust them enough to make a decision, it signals that the entire working relationship will be plagued by mistrust and second-guessing.

Good agencies value partnership and trust above all else. They want to work with clients who are confident in their expertise and are ready to collaborate. A request for a reference says, "I don't believe anything you've shown me." Furthermore, they respect their existing clients' time and won't bother them every time a new prospect wants a chat. Their work, demonstrated through case studies and the free audit, should speak for itself. If it doesn't, then they aren't the right fit for you, but the issue isn't a lack of references, it's a failure in their vetting process. You either beleive in their ability to do the job or you don't. There's no middle ground.

Is an agency even the right choice for you?

Before you go all-in on finding an agency, it's worth taking a step back and asking if it's the right model for your business right now. You generally have three options for managing your ad spend, each with its own pros and cons for a London-based business.

An agency is the best fit when you have a significant ad spend (£5k+/month) and need ongoing, expert management and optimisation but don't want the overhead of a full-time hire. If your budget is smaller or you just need strategic guidance to get started, a consultant might be a better first step. If you have a massive budget and advertising is core to your business, an in-house team might make sense in the long run. Making the right choice between an agency, consultant, or in-house team is a critical first step.

I've put together my main recommendations into a table below to give you a clear, actionable checklist for your search.

Choosing an ad agency in London is one of the most significant marketing decisions you'll make. Getting it right can transform your business. Getting it wrong can set you back months and cost you tens of thousands in wasted fees and ad spend. Don't rush it. Follow a rigorous process, trust your gut, and prioritise genuine expertise over slick salesmanship. The right partner is out there, but you have to know how to look for them.

This process can be a lot to handle on your own. If you want a second opinion or would like to see what a proper, no-strings-attached audit looks like, we offer a free initial consultation. We'll get on a call, review your strategy and accounts together, and give you a taste of the expertise you should expect from a true growth partner. Feel free to reach out to schedule your free session.