London's Best Ad Experts: The Founder's Vetting Guide

Finding a paid ads agency in London feels like trying to find a decent pint for under a fiver. There's a pub on every corner, but most of them are serving up disappointment. You're a founder, you're busy, and you're getting bombarded with LinkedIn messages from 'growth hackers' who were selling crypto last year. You've got a business to run, and you need someone who actually understands that your goal isn't just 'more clicks'—it's more profit in the bank.

The truth is, most agencies are brilliant at selling themselves and pretty average at selling their clients' products. They'll show you glossy reports filled with vanity metrics and talk about 'brand awareness' until you're blue in the face. But when you look at your bottom line, nothing's changed. The problem isn't that you can't find an agency; it's that you're struggling to find one that ties its work directly to your business goals. Let's sort that out.

So, why do so many London agencies get it wrong?

It usually comes down to a few home truths they don't want you to know. First, the person who sells you the dream on the sales call is almost never the person who will actually be running your campaigns. You get the seasoned director for the pitch, and the keys to your ad account are handed to a junior account manager who's barely out of uni and is juggling 15 other clients. They follow a template, they don't have the time or experience to properly understand the nuances of your business, wether you're a FinTech startup in Canary Wharf or a B2B SaaS in the Silicon Roundabout.

Second, they're obsessed with the wrong things. They focus on what's easy to measure and looks good on a graph, like impressions, click-through rates (CTR), and reach. Does any of that pay your staff's wages? No. It's noise. It’s the advertising equivalent of looking busy. A good agency obsesses over what you obsess over: Cost Per Acquisition (CPA), Customer Lifetime Value (LTV), and Return on Ad Spend (ROAS). They should be talking about your sales funnel, your margins, and your customer churn rate. If they don't ask about this stuff in the first call, it's a massive red flag. They're not thinking like a business partner; they're thinking like a media buyer who just needs to spend your money.

I remember one B2B software client who came to us after burning through £50k with a well-known agency. The agency's reports were beautiful. Lots of upward-trending charts for 'engagement'. But they'd generated a grand total of three qualified leads. Three. The agency had never bothered to ask about the client's incredibly long sales cycle or the fact their ideal customer was a C-level executive who doesn't click on flashy Facebook ads. It's a classic case of failing to translate business goals into a real strategy. Learning how to properly vet a B2B agency in London can save you that kind of headache.

Before you even look at an agency's website, what's your number?

Right, this is the bit where you have to do some homework. You can't judge an agency's potential ROI if you don't know what a customer is actually worth to you. If you walk into a conversation without knowing your numbers, you're flying blind, and you'll have no way to know if the results they're promising are good, bad, or completely irrelevant. The most important number is your Lifetime Value (LTV).

It's not as complicated as it sounds. Here's the simple maths:

• Average Revenue Per Account (ARPA): What's a customer worth to you each month on average?
• Gross Margin %: After your cost of goods/service, what's your profit margin?
• Monthly Churn Rate %: What percentage of customers do you lose each month?

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

Let's say you run a SaaS business. Your average customer pays £200/month, your gross margin is 80%, and you lose 5% of your customers each month. Your LTV would be (£200 * 0.80) / 0.05 = £3,200. This means, on average, each new customer you acquire is worth £3,200 in gross margin to your business over their lifetime.

Now you've got a benchmark. A healthy business model often aims for a 3:1 LTV to Customer Acquisition Cost (CAC) ratio. So, for a £3,200 LTV, you can afford to spend around £1,066 to acquire a new customer and still have a very profitable model. This number is your weapon. It's what allows you to have an intelligent conversation with an agency about what a realistic CPA target should be. It stops you from panicking if a lead costs £100, because you know it's part of a much bigger, profitable picture.

To make it even easier, here's a calculator. Play around with your own numbers.

How do you actually spot a genuinely good agency?

Okay, you've got your LTV number. Now you're ready to start looking. But how do you seperate the wheat from the chaff? It's all about looking for proof, not promises. Any agency can say they "drive results." You need to find one that can prove it, ideally in a niche that's similar to yours.

-> Dig into their Case Studies: Don't just skim them. Read them properly. Are they specific? Do they mention the actual client? Do they talk about the initial problem, the strategy they implemented, and the tangible business results (e.g., revenue, ROAS, qualified leads)? Vague claims like "increased brand awareness by 200%" are worthless. I've seen some of our own case studies get incredible results, like taking a medical job matching SaaS from a £100 CPA down to just £7, or generating $115k in revenue for a course creator in six weeks. Those are real, specific outcomes. That's what you're looking for. If their case studies are all locked behind a "contact us for more info" wall, it's often because they don't have anything impressive to show.

-> The Discovery Call is Your Interview: When you get on a call, your job is to listen for how they think. Don't just let them pitch you. Ask them hard questions. A great one is: "Based on what you've seen of our business so far, what's the first thing you'd change about our current strategy?" A bad agency will give you a generic answer about "optimising your campaigns." A great agency will have done their homework, looked at your website, maybe even your existing ads, and will give you a specific, insightful, and possibly even uncomfortable critique. They should be challenging you, not just telling you what you want to hear. If it feels like they truly understand your business after just 30 minutes, that's a brilliant sign.

-> Reviews and Reputation: Check their reviews on sites like Clutch or Google. But don't just look at the star rating. Read what clients are actually saying. Are they talking about the agency being a genuine partner? Do they mention specific results? Or is it all fluffy praise about them being "great to work with"? You're looking for evidence of impact. To be honest, if you've seen their detailed case studies and had a great discovery call where they've demonstrated their expertise, asking for references is often a sign of mistrust. A good agency has already given you all the proof you should need. This is a core part of the framework for choosing the right agency from the start.

To help you structure this, here's a simple flowchart for the vetting process. It breaks down the steps from initial research to making a final decision.

What about the cost? Is the 'London Premium' real?

Yes, London is an expensive city, and that often translates to higher agency fees. But the real question isn't "how much does it cost?" but "what's the value?" A cheap agency that gets you no results is infinitely more expensive than a pricier one that delivers a 5x return on your investment. Understanding how London agency pricing relates to ROI is absolutely vital.

Agency fees in London typically fall into three buckets:

• Monthly Retainer: A fixed fee each month, regardless of ad spend. This is common and provides predictability. For a decent agency in London, you should expect this to start at £2,000-£3,000 per month and go up significantly from there.
• Percentage of Ad Spend: The agency takes a cut of what you spend on ads, usually 10-20%. This model can work, but it can also incentivise the agency to simply spend more of your money, not necessarily more effectively.
• Performance-Based: A lower retainer plus a bonus based on hitting certain targets (e.g., a percentage of revenue generated). This can be great for aligning incentives, but not all agencies offer it, especially for new clients.

What you pay in fees is only one part of the equation. You also need to budget for the actual ad spend. A common mistake founders make is hiring an agency and leaving no budget for the ads themselves. The agency can't make magic happen without fuel for the fire. Your total investment is always `Agency Fee + Ad Spend`.

Here’s a rough idea of what you might expect to invest in total per month for different levels of business in London. This isn't gospel, but it's a realistic starting point.

The key takeaway is that you need to be realistic. If you only have a £1,000 total monthly budget, you might be better off learning to run the ads yourself or hiring a freelancer rather than a full agency. Making the choice between a PPC agency and an in-house team is a major financial decision for UK founders, and you need to be properly capitalised to make an agency partnership work.

Your Final Checklist Before Signing a Contract

You've done the research, you've had the calls, and you've found an agency you think is the one. Before you sign on the dotted line, run through this final checklist. This is the culmination of everything we've discussed, designed to be your final sanity check.

I've detailed my main recommendations for you in a table below to make it as actionable as possible:

Choosing an agency is a big decision. It's not just a line item on your P&L; it's a partnership that can make or break your growth trajectory for the next year. Taking the time to do this vetting process properly is the best investment you can make. It's the difference between finding a team that simply spends your money and one that multiplies it.

If you're tired of the agency carousel and want to speak with a team that thinks about business goals first and ads second, you might find our approach refreshing. We offer a completely free, no-obligation strategy session where we'll look at your business, your goals, and your current campaigns, and give you honest, actionable advice you can use, whether you decide to work with us or not. It's a chance to see how a genuine expert partner thinks.