London's Paid Ads Agencies: The Founder's Vetting Guide

Right then. You're looking for a paid ads agency in London. Good luck. The city is crawling with them, from massive glass-and-steel operations in Canary Wharf to two-person outfits in a Shoreditch co-working space. And most of them will tell you the exact same thing, using the exact same slides, promising the exact same results. It's a proper minefield.

The problem is, most founders and marketing heads approach this process backwards. They start by comparing agency websites and case studies, getting lost in a sea of logos and vague promises of "skyrocketing your ROI". This is the wrong way to go about it. The secret to finding the right partner isn't about them, it's about you. It's about doing the hard work upfront so you can walk into those meetings, virtual or otherwise, armed with the clarity to see straight through the waffle.

This isn't just another list of agencies. This is a playbook. This is the framework we've seen work time and again for businesses trying to navigate the crowded London market. Forget what you think you know about hiring an agency. Let's start from the beginning.

So, who are you actually selling to? Hint: it's not a demographic.

Before you even think about writing an ad or briefing an agency, you need to get this bit right. Forget the sterile, demographic-based profile your last marketing hire made. "Companies in the finance sector with 50-200 employees" tells you nothing of value. It's lazy, and it leads to generic ads that speak to no one. To stop burning cash, you must define your customer by their pain.

You need to become an expert in their specific, urgent, expensive, career-threatening nightmare. 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. For a legal tech SaaS, the nightmare isn't 'needing document management'; it's 'a partner missing a critical filing deadline and exposing the firm to a malpractice suit.' Your Ideal Customer Profile (ICP) isn't a person; it's a problem state. What keeps them awake at night? What problem, if solved, would make them look like a hero to their boss?

Once you've isolated that nightmare, you can find them. Where do they hang out online when they're not working? Find the niche podcasts they listen to on their commute, like 'Acquired'; the industry newsletters they actually open, like 'Stratechery'; the SaaS tools they already pay for, like HubSpot or Salesforce. Are they members of the 'SaaS Growth Hacks' Facebook group? Do they follow people like Jason Lemkin on Twitter? This intelligence isn't just data; it's the blueprint for your entire targeting strategy. Do this work first, or you have no business spending a single pound on ads, whether you're doing it yourself or looking for the best ad agency for your startup.

Do you actually know your numbers?

Here's the second piece of homework. 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 its counterpart: Lifetime Value (LTV). If you don't know this number, you're flying blind, and you're at the mercy of any agency that throws a low CPL figure at you, without any context of lead quality.

Here's the simple maths. It's not complicated, but it's probably the most important calculation you'll do all year.

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

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

So, in that example, each customer is worth £10,000 in gross margin. A healthy 3:1 LTV:CAC (Customer Acquisition Cost) ratio means you can afford to spend up to £3,333 to acquire a single customer. If your sales process converts 1 in 10 qualified leads into a customer, you can afford to pay up to £333 per qualified lead. Suddenly, that £250 lead from a CTO on LinkedIn doesn't seem expensive, does it? It looks like a bargain. This is the maths that unlocks aggressive, intelligent growth. Knowing your Google Ads budget and potential ROI is the first step before you even start looking for help.

What type of London agency are you actually talking to?

Once you've done your homework, you can start looking outwards. Most agencies in London, despite their fancy websites, fall into one of three buckets. Understanding these archetypes is crucial for filtering out the noise.

1. The Factory
These are the big names, often part of a global network, with impressive offices and a long list of FTSE 100 clients. They run on process. Your account will be managed by a junior account manager, who acts as a go-between for you and the actual specialists (who you'll probably never speak to). They are good at executing large, straightforward campaigns for established brands. They are generally not good for startups or scale-ups that need agility, deep strategic input, and direct access to senior experts. You'll become a small fish in a very large, impersonal pond.

2. The Boutique (or Specialist)
This is typically a smaller, owner-led agency. They are experts in one or two things – maybe B2B SaaS on LinkedIn, or DTC eCommerce on Meta. The person who sells you the service is often the person who will be running your campaigns. What they lack in size, they make up for in depth of knowledge and direct accountability. I've seen specialists achieve incredible results, like taking a medical job matching SaaS from a £100 CPA down to just £7. The downside? They might not have expertise across every platform, and their capacity is limited. This is often the best fit for businesses that know exactly what they need and want a true partner.

3. The 'Full-Service' Mirage
This is the most dangerous one. They claim to do everything: SEO, content, social media management, email, web design, and paid ads. The reality is they're a master of none. Paid advertising is a deep specialism that requires constant learning and focus. An agency that also offers to write your blog posts is unlikely to have a genuine expert running your Google Ads account. They often sell a bundled dream but deliver mediocre results across the board. For performance-focused advertising, avoid them. You're not buying a Swiss Army knife; you're hiring a surgeon.

How to see through the sales pitch: The Vetting Framework

You've done your homework, you've identified a few specialist agencies that look like a good fit. Now comes the call. This is where you separate the real experts from the smooth talkers. You need to ask better questions. Here’s a framework.

Instead of: "What's your experience in B2B?"
Ask: "Show me a case study for a B2B SaaS client in the FinTech space with a similar annual contract value to ours. Can you walk me through the exact strategy you used on LinkedIn Ads, why you chose that approach, and what the CPL was?"
Why it's better: It forces them to be specific. A good agency will relish this question. They'll talk about how they targeted decision makers, maybe using company lists from tools like Apollo.io, and then tested different ad formats like video versus image ads with Lead Gen Forms. We've run campaigns for B2B software where we've hit a $22 CPL for senior decision makers on LinkedIn – they should have similar, tangible examples.

Instead of: "What results can you get me?"
Ask: "What would your process be for the first 90 days with our account? What are the key milestones, and what data will you use to make optimisation decisions? What does your reporting look like?"
Why it's better: This tests their process, not their promises. Guarantees are a huge red flag. No one can promise results. A real pro will talk about an initial audit phase, a testing structure for creative and audiences, and a clear reporting cadence that focuses on business metrics (like cost per qualified lead), not vanity metrics (like impressions).

Instead of: "Who will work on my account?"
Ask: "Can I have a brief chat with the actual person who will be managing our campaigns day-to-day? I'd love to hear about their direct experience with Google Ads for high-ticket services."
Why it's better: It cuts through the agency hierarchy. You want to know if the person with their hands on the buttons is a seasoned expert or a recent graduate. In many larger agencies, the impressive senior person who sells you the dream hands it off to a junior who you never meet. Insist on speaking to the practitioner. If you're looking for help from a London Google Ads expert for your B2B SaaS, you need to ensure you're actually getting an expert.

Instead of: "Can you send me some references?"
Ask: "Your case studies look interesting. Can you explain the main challenge in the [eCommerce Cleaning Products] campaign and how you specifically drove that 633% return?"
Why it's better: Tbh, asking for references after you've seen detailed case studies and had a deep strategy call can be a red flag for the agency. It signals a lack of trust from the start. A good case study should contain all the proof you need. A better approach is to grill them on the details of their public successes. It shows you've done your research and forces them to demonstrate their strategic thinking, not just present a glossy PDF.

When to run a mile: The big red flags

Some things should be an instant deal-breaker. If you see any of these, politely end the conversation and move on. Your time and money are too valuable.

• Guarantees: As mentioned, anyone who "guarantees" a specific ROAS or CPA is either lying or inexperienced. Paid advertising is a dynamic system, not a vending machine.
• Long, Inflexible Contracts: A confident agency doesn't need to lock you into a 12-month contract from day one. Look for a 3-month initial term or a 30-day notice period. They should be confident enough in their results to keep you as a client.
• Opaque Pricing or Reporting: You should know exactly what you're paying them and have full, admin-level access to your own ad accounts. If they're secretive about either, they're hiding something. It's that simple.
• The High-Pressure Close: "This special offer is only valid if you sign today." This is a classic sales tactic, not the start of a strategic partnership. A good agency will give you the space to make the right decision.
• Lack of Curiosity: Did they ask you about your LTV, your sales cycle, your best customers? Or did they just launch into a pitch about their "proprietary process"? If they aren't deeply curious about your business, they can't possibly market it effectively.

Making the right choice between DIY and hiring an agency is a major decision, and these red flags can help you avoid a costly mistake.

The final test: a genuinely useful conversation

Ultimately, the best way to vet an agency is to get a taste of their expertise. This is why many good agencies, including us, offer a free initial consultation or account audit. But beware – not all 'free audits' are created equal.

A bad one is just a thinly veiled sales pitch. They'll run your website through an automated tool, show you some scary-looking red marks, and then spend 30 minutes talking about themselves. It provides no real value.

A good one is a proper strategy session. The consultant will have reviewed your account beforehand. They'll share their screen, walk you through your actual campaigns, and provide specific, actionable advice you can implement right away, whether you hire them or not. They'll ask smart questions and challenge your assumptions. You should leave the call feeling like you've learned something and have a clearer path forward. This is the single best indicator of what it would be like to work with them. It proves their expertise, builds trust, and shows they're genuinely invested in helping you succeed, not just in closing a deal.

Choosing an agency in London is a massive decision. It's a proper partnership that can make or break your growth for the next year. Don't rush it. Do your homework. Ask tough questions. And trust your gut. Look for the specialists who are as obsessed with your business metrics as you are. They're out there, you just have to know how to find them.

What should you expect to pay?

Agency fees in London can vary wildly. It's important to understand the different models and what's reasonable for your level of ad spend. Most specialist agencies will use a combination of a flat retainer and a percentage of ad spend, especially as you scale.

I've detailed my main recommendations for you below:

Ultimately, the journey to finding the right paid ads partner is one of the most important you'll take as a founder. It's a complex decision, and sometimes you need an expert eye to help you see things clearly. If you've done the homework and are still struggling to compare your options, or if you want a no-nonsense second opinion on your current advertising efforts, that's what we're here for. We offer a completely free, no-obligation strategy session where we can dive into your specific challenges and give you some actionable advice.

Hope this helps!