London's Ultimate Guide to Vetting Ad Agencies

Right then. You're based in London and trying to find a social media agency. I'm sorry. It's a jungle out there, full of slick salespeople in trendy offices who are brilliant at selling their own services and absolutely pants at selling yours. The problem isn't finding an agency; they're on every corner from Canary Wharf to Richmond. The problem is finding one that won't just burn your cash with a smile.

Most founders make the same mistake. They look at an agency's own social media, see a polished feed, and assume they can do the same for them. That's like hiring a decorator because they wear nice clothes. It's irrelevant. You need to look under the bonnet, kick the tyres, and understand if they're actually mechanics or just car salesmen. And frankly, most of them are just salesmen.

This isn't another generic checklist. This is a framework for vetting agencies in the uniquely competitive London market, built from years of seeing what actually works versus what just sounds good in a proposal. Forget the fluff. Let's talk about what really matters.

So, what should I actually be looking for in their case studies?

This is your single most important filter. Before you even think about booking a call, you need to dissect their past work. But most people do this wrong. They see a big brand logo and get impressed. Don't. A huge brand can get results with a monkey running the ads because they already have a massive tailwind of brand recognition. You probably don't have that luxury.

You need to find proof that they understand your type of business. If you're a B2B SaaS company, you couldn't care less that they got 10 million views for a luxury fashion brand's launch. It's a completely different skillset. You need to see evidence they can generate high-quality leads for a complex product. I'm talking about specific results, like a campaign we ran that brought in B2B decision-maker leads for $22 a pop on LinkedIn, or another that generated 1,535 trials for a SaaS client. Those are real business metrics, not fluffy 'engagement' numbers.

Similarly, if you run an eCommerce store, you want to see proof of return on ad spend (ROAS). Don't accept "we increased sales." Demand the numbers. Did they get a 618% ROAS? A 1000% return? We've managed campaigns that hit these numbers, and it's because we focused on the metrics that matter. A good case study should tell you the niche, the platform, the objective, and the hard numbers. If it's just a glossy PDF with vague statements like "increased brand awareness," bin it. It's marketing fluff designed to hide a lack of real results.

And be realistic. Paid advertising isn't magic. If an agency's case studies all look too good to be true, they probably are. Or, they're only showing you the one freak success they had out of a hundred failures. Look for consistency and relevance. A solid, believable result for a business just like yours is worth a hundred unbelievable claims for a business you have nothing in common with.

This simple process will help you filter out at least 80% of the agencies in London before you even waste a minute on a call. You're looking for a partner who has already solved the problems you are currently facing. Their track record is the only thing you can rely on.

How do I spot real expertise on a discovery call?

Alright, so you've found an agency with some promising case studies. Now comes the real test: the discovery call. This is where you separate the experts from the script-readers. A bad agency will spend 45 minutes talking about themselves, their 'process', their awards, and their team. A good agency will spend 45 minutes interrogating you.

Their goal shouldn't be to sell you; it should be to diagnose your business. If they aren't asking you probing, sometimes uncomfortable questions, they're not trying to solve your problem. They're just trying to close a deal.

What are these questions? They should be digging into the core economics of your business.

• -> "What's your customer lifetime value (LTV)?"
• -> "What's your current customer acquisition cost (CAC), and what's your maximum allowable CAC?"
• -> "What's your monthly churn rate?"
• -> "What's your sales conversion rate from lead to customer?"

Why do these matter? Because without this information, it's impossible to build a profitable advertising strategy. An agency that doesn't ask for this is planning to fly blind with your money. I remember one potential client who was fixated on getting a low Cost Per Lead (CPL). But after we calculated their LTV, it turned out they could afford to pay ten times their target CPL and still be wildly profitable for each customer they acquired. A good agency finds that leverage. A bad one just focuses on a vanity metric.

You should also be asking them tough questions. Don't ask "what results can you guarantee?" It's a silly question because no one can guarantee anything, and anyone who does is lying. Instead, ask strategic questions:

• -> "Given our target customer is a Head of Sales in UK tech companies, what initial audiences would you test on LinkedIn, and why?"
• -> "Our main competitor is X. How would you position our ads to differentiate us?"
• -> "We get clicks but not enough conversions. What are the first three things you would investigate on our landing page?"

Listen to their answers. Are they specific? Do they show a deep understanding of the platform's mechanics and your business context? Or are they waffly and full of jargon? Real expertise is quiet confidence and specific advice. A lack of expertise is loud, full of buzzwords, and makes grand, vague promises. Many founders find their website gets traffic but it just doesn't convert. To fix this, you need a deep look at the alignment between your ad copy, your targeting and your landing page, which is a problem a good performance marketing agency can help solve.

What sort of budget do I really need for a London agency?

Let's talk money. This is where a lot of businesses in London get it wrong. They either try to cheap out and hire a bargain-basement agency (who will deliver bargain-basement results) or they get taken for a ride by a big-name agency with huge overheads.

First, the agency fee. In London, for a competent agency that knows what they're doing, you should expect to pay a monthly retainer of anywhere from £2,000 to £10,000+, depending on the scope of work. Some will work on a percentage of ad spend, usually 15-20%, but often with a minimum retainer. If someone quotes you £500 a month, run a mile. You'll be getting a junior account manager who spends 2 hours a month on your account, tops. You get what you pay for, and cheaping out on the expertise that manages your ad spend is the most expensive mistake you can make.

Second, and more important, is your ad spend budget. An agency is useless without enough fuel for the fire. Starting with £500/month in ad spend in a competitive market is like trying to boil the ocean with a kettle. You won't get enough data to make any meaningful decisions. For a service-based business, as a bare minimum, I'd recomend starting with £1,000-£2,000 a month. We've seen campaigns for home cleaning services generate leads for £5 each, meaning £1,000 could get you 200 leads. But for a more competitive space like HVAC services, leads can be $60 each, so you'd only get 16 leads for the same spend. You need a budget that allows for proper testing and optimisation.

For eCommerce or SaaS, it's a different calculation based on your product price and LTV. But the principle is the same: you need to spend enough to get statistically significant data. If your Cost Per Purchase is £25, you need to spend at least £75-£100 per ad set just to see if it's capable of getting a few sales. If your budget is too small, you'll turn ads off before they ever had a chance to work. A good agency will give you an honest assesment of the budget required to actually hit your goals. A bad one will take whatever you're willing to give them, knowing full well it's not enough to succeed.

What are the big red flags I need to avoid?

Your bullshit detector needs to be finely tuned during this process. Agencies are notoriously good at sounding plausible. Here are the clear, unmissable red flags that should have you running for the hills.

1. The "Guarantee". As I mentioned, this is the biggest one. If you hear the words "we guarantee you'll get a 5x ROAS" or "we guarantee 100 leads a month," end the call. They are either lying or incompetent. Paid media is an auction, and there are far too many variables to guarantee anything. Experts talk in probabilities and benchmarks, not certainties.

2. The "Secret Sauce". When you ask about their strategy and they reply with vague nonsense like "we use our proprietary optimisation method" or "it's our secret sauce," it means they don't have a strategy. They're just pressing buttons in the ad manager and hoping for the best. A real expert will happily walk you through their thinking. They'll talk about audience testing structures, creative iteration processes, and funnel optimisation. It's not a secret; it's a craft. I'd particularly look for an agency that understands the full funnel, because simply getting clicks is easy, but if you need help turning those clicks into actual customers, a specialist is your best bet and you might find our UK B2B agency vetting guide helpful here.

3. The "Awareness" Obsession. Be very wary of any agency that immediately suggests running "Brand Awareness" or "Reach" campaigns. As I've said before, you're literally paying platforms like Meta to find the people least likely to ever buy from you, because their attention is cheap. For 99% of businesses, awareness is a byproduct of sales, not a prerequisite. You want an agency focused on conversion objectives—leads, sales, sign-ups. If their first thought is 'awareness', it often means they don't know how to drive actual business results.

4. The Bait and Switch. This is classic. You have a fantastic call with a senior strategist or the agency founder who sounds like they know everything. You sign the contract. Then, on day one, you're handed over to a junior account manager fresh out of uni who has never managed a budget bigger than their lunch money. You must ask: "Who will be my day-to-day contact, and what is their direct experience running campaigns for businesses like mine?" If you can't get a straight answer, it's a bad sign.

5. A Reluctance to Talk About Failure. Ask them about a campaign that went wrong and what they learned from it. If they claim they've never had a failing campaign, they are lying. Every single person in paid ads has launched campaigns that have flopped. It's part of the process. An expert will see it as a learning opportunity. They'll be able to tell you, "We tested this angle, the data showed it wasn't working because of X, so we pivoted to Y and saw a 30% improvement." That's the answer of a professional. Someone who can't admit to failure is someone who doesn't know how to learn from it.

Finding the right agency in London is tough, but it's not impossible. You have to be more disciplined and rigorous than the other founders they speak to. Do your homework, trust the numbers over the sales pitch, and you'll find a partner that can actually help you grow.

So how do I make the final decision?

After you've done your research, reviewed the case studies, and had the discovery calls, you need a way to compare your options objectively. It's easy to be swayed by a slick presentation or a charismatic salesperson. You need to strip all that away and score them on what actually matters.

This isn't about who you 'like' the most. It's about who you trust to be a steward of your capital and a driver of your growth. It's one of the most important hiring decisions you'll make. Don't take it lightly.

I've detailed my main recommendations for you below in a simple framework. Use it to score each agency you're seriously considering. Be brutally honest in your scoring. This will make your final decision much, much clearer.

Choosing an agency is a huge drain on your most valuable resource: your time. It involves hours of research, multiple meetings, and a lot of uncertainty. Even after all that, you can still get it wrong, which means months of wasted ad spend and management fees before you realise you're back at square one. For many founders, this process is so daunting they either make a bad choice quickly or suffer from analysis paralysis and do nothing.

Working with a proven expert from the start shortcuts this entire painful process. Instead of vetting ten agencies, you're tapping into the expertise of a team that has already navigated these waters hundreds of times. You get straight to the strategy and execution, focusing your time on running your business, not on managing an underperforming agency.

If you're a London-based founder and you're tired of the agency circus, perhaps we should talk. We offer a free, no-obligation 20-minute strategy session where we can look at what you're doing now and give you some actionable advice. No hard sell, no fluffy promises. Just straightforward, expert opinion to help you get some clarity.