London Ad Agencies: The Founder's Vetting Framework

I see this question a lot. The London agency scene is a proper mess of buzzwords, big promises, and fancy Shoreditch offices. It's almost impossible to tell who's genuinely good and who's just good at selling themselves. You're right to be cautious. Most businesses I talk to have been burned at least once before they come to us.

The truth is, vetting an agency isn't about their awards or their client list. It's about a series of targeted tests designed to expose their real level of thinking. Here’s a no-nonsense framework to cut through the rubbish and find a partner that can actually move the needle for your business.

So, how do you find the right people?

First things first, you need to forget the conventional wisdom. Most founders start by looking at an agency's website, their list of clients, maybe some testimonials. This is the wrong place to start. It tells you what they *want* you to see, not what you *need* to know. Instead, the entire vetting process should revolve around one single concept: their understanding of your customer's pain.

Forget the sterile, demographic-based profile your last marketing hire made. "Companies in the finance sector in the City of London with 50-200 employees" tells you precisely nothing of value. It leads to generic ads that get ignored. To stop burning cash, you must define your customer by their specific, urgent, expensive, career-threatening nightmare. That's your Ideal Customer Profile (ICP).

Your Head of Engineering client isn't just a job title; she's a leader terrified of her best developers quitting out of frustration with a broken workflow. Your target at a law firm isn't just 'a partner'; it's someone petrified of missing a critical filing deadline and exposing the firm to a malpractice suit. Your ICP isn't a person; it's a problem state.

This is your first filter. When you get on a call with a London agency, your first question shouldn't be "What's your process?". It should be, "Here's our customer. Tell me about their biggest nightmare, and where would you find them online when they're not thinking about work?".

If they start talking about broad demographic targeting on Facebook, they've failed. If they start talking about the niche podcasts they listen to on their commute on the Tube, the industry newsletters they actually open, the SaaS tools they already pay for, or the private LinkedIn groups they're in... now you're talking to a professional. This intelligence is the blueprint for a winning strategy. Any agency that hasn't done this work has no business spending a single pound of your money.

How do you actually check their track record?

Okay, so they've passed the first test. They seem to get your customer. Now you look at their case studies. But not in the way most people do. Don't be impressed by big brand logos. A campaign for a massive company like Tesco or Barclays has very different goals and resources than a campaign for a growing London startup. You need to find proof that they've solved problems similar to yours.

Look for case studies in your niche or a closely related one. And look for real numbers, ideally in pounds. A "1000% Return On Ad Spend" sounds great, but it's meaningless without context. I remember we achieved that for a subscription box client, but that result is specific to their margins, audience, and offer. It doesn't mean we can do it for everyone. Be realistic.

When you find a relevant case study, your job is to pick it apart. Get them on a call and ask brutally specific questions:

• -> "Walk me through the first 30 days of that campaign. What was the first thing you tested?"
• -> "What was your biggest assumption going in that turned out to be completely wrong?"
• -> "Why did you choose LinkedIn Ads for them instead of Google? Be specific."
• -> "What was their Customer Acquisition Cost (CAC) and what did you estimate their Lifetime Value (LTV) to be?"
• -> "The case study says you achieved a £7 CPA. What was it when you started?" (For context, we took one medical SaaS client from a £100 CPA down to £7, but that journey is the important part).

Their answers will tell you everything. A good partner will be honest about the failures and the learning process. A salesperson will just repeat the glossy headline from the case study. Tbh, a lot of founders struggle with this part, which is why we've put together a more detailed guide on how to differentiate between agencies that seem similar on the surface.

And a note on trust. If you've reviewed their case studies, had a deep strategy call, and they've opened up their process to you, asking to speak to one of their current clients is a massive red flag for the agency. It signals you don't trust them, and the relationship is probably doomed from the start. A good agency-client relationship is a partnership, and it has to start with mutual trust.

What's the one test every agency must pass?

This brings us to the most critical part of the process: the free audit or strategy session. Any agency worth their salt will offer this. We do it for every potential client. It's not a sales call; it's a working session. It’s your chance to see how they think in real-time, applied to your actual business.

This is where you bring your biggest challenges. Show them your ad account if you have one. Tell them what's not working. Your goal is to walk away with at least one, tangible, high-impact idea that you can implement yourself. If you don't, they are not the right partner. They should be challenging your assumptions, not just nodding along and telling you what you want to hear.

A top-tier agency will look at your business and immediately start questioning the core components. They wont just talk about keywords or audiences; they'll talk about your offer.

The "Request a Demo" button on your website? It's probably the most arrogant, high-friction Call to Action ever conceived. It presumes your prospect, a busy decision-maker, has nothing better to do than book a meeting to be sold to. A great agency will tell you this. They'll push you to replace it with something that provides undeniable, instant value. For a SaaS founder, that's a free trial or a freemium plan. Let the product do the selling. For a service business, it might be a free, automated audit tool, a valuable checklist, or a short video course. For us, it's this exact kind of no-fluff advice and a free strategy session where we audit failing campaigns. You must solve a small, real problem for free to earn the right to solve their bigger problems for a price.

How do you know if you can afford them?

This is the question that stops most founders in their tracks. They see an agency's monthly retainer – say £3,000 or £5,000 – and immediately think "that's too expensive". This is the wrong way to think about it. The real question isn't "How low can my Cost Per Lead go?" but "How high a CPL can I afford to acquire a truly great customer?" The answer lies in your Customer Lifetime Value (LTV).

Before you talk to any agency, you need to know this number. If you dont know it, you are flying blind. Let's break it down simply:

• -> Average Revenue Per Account (ARPA): What do you make per customer, per month?
• -> Gross Margin %: What's your profit margin on that revenue?
• -> Monthly Churn Rate: What percentage of customers do you lose each month?

The calculation is: LTV = (ARPA * Gross Margin %) / Monthly Churn Rate

For example: (£500 ARPA * 80% Gross Margin) / 4% Monthly Churn = £10,000 LTV.

Suddenly, that £3,000 retainer doesn't look so scary. With a healthy 3:1 LTV:CAC ratio, you can afford to spend up to £3,333 to acquire a single customer. The agency only needs to bring you one new customer a month to be profitable. This is the maths that unlocks aggressive, intelligent growth. Use the calculator below to find your own numbers.

Once you know your LTV, you can assess an agency's pricing. Most London agencies work on a few models: a fixed monthly retainer, a percentage of ad spend, or (rarely) a performance-based deal. There's no single "best" model; it depends on your stage. Early on, a fixed retainer gives you predictable costs. As you scale, a percentage of spend might make more sense. The important thing is that it aligns with your goals and your LTV can support it. For a more detailed breakdown, we wrote a guide on how to understand London ad agency pricing.

What are the final red flags to watch out for?

By this point, you should have a very short list of potential partners. The final decision often comes down to gut feel, but there are some clear red flags that should make you run for the hills.

1. They Guarantee Results: Anyone who guarantees a specific ROAS or number of leads is either lying or inexperienced. Paid advertising is a dynamic system of testing and optimisation. There are no guarantees.

2. Their Strategy is a "Secret Sauce": If they can't clearly explain their process, their targeting logic, and their optimisation strategy in plain English, they don't have one. They're just pushing buttons and hoping for the best.

3. You Don't Speak to the Strategist: You get sold by a slick sales director, then handed off to a junior account manager. Insist on speaking with the actual person who will be managing your account and building your campaigns. Their expertise is what you're paying for.

4. They Pressure You: "This offer is only good until Friday." "We only have one spot left." These are cheap sales tactics. A true partner agency will give you the space to make the right decision.

Choosing an agency is one of the most important marketing decisions a founder can make. It's not just about hiring a supplier; it's about finding a strategic partner who is as invested in your growth as you are. Whether you're specifically looking for a B2B ad agency in London or a broader consumer-focused one, this framework will help you look past the sales pitches and identify true expertise.

The goal isn't just to get more clicks or leads. The goal is to build a predictable, scalable customer acquisition engine that fuels your company's growth for years to come. That requires a level of strategic thinking that most agencies simply don't possess. Do the work upfront to find one that does.

This is the main advice I have for you:

Finding the right agency is a massive lever for growth, but picking the wrong one can set you back months and cost you a fortune. It's worth taking the time to do it properly. If you're looking for an experienced team to partner with, we offer a completely free, no-obligation strategy session where we'll do a deep dive into your business and give you a clear plan of action. Feel free to get in touch if you'd like to see how we think.

Hope that helps!