Top 25 Big six advertising agencies based in London, United Kingdom

How this list was built

Company categories are rarely black and white. RevenueBase reads each company's own description of its business to decide whether it belongs among big six advertising agencies, so some rows will be adjacent to the category rather than pure plays. The description and RevenueBase Smart Search rating on every row are there so you can check each match yourself.

Source

The RevenueBase company graph: 60 million companies and 398.9 million professional contacts resolved from primary sources, queried with RevenueBase Smart Search, the natural-language company search available to every RevenueBase user.

What qualifies

Companies headquartered in London, United Kingdom whose own description of what they do matches “big six advertising agencies”. Matching reads each company's description, not its industry code or its name.

RevenueBase Smart Search

RevenueBase Smart Search is how this list is assembled and how each row is rated. RevenueBase's data engine and AI assess a company's LinkedIn profile, information supplied directly by the company, its website and other publicly available information, then relate it to a category defined in a user's own words. Each row is rated strong evidence (its own description names big six advertising agencies), partial evidence (it names part of the category) or indirect evidence (the description shown names none of it and the match rests on the rest of the company's profile). The description supporting each rating is quoted on the row.

What we exclude

Companies whose own description does not clearly place them among big six advertising agencies, and records with no verified work email. Companies headquartered outside London are excluded even when they keep an office there.

Match quality

In blind human judging of this matching method, 86% of matched companies were on target. Expect roughly 1 in 7 rows to be adjacent to the category rather than a pure play. Every row shows the company's own description and its RevenueBase Smart Search rating so you can judge each match yourself. Read the full methodology: how the lists are built, how the judging worked, and what the figure means.

Ranking

Team size, largest first: companies are ordered by the number of professionals RevenueBase tracks at each one. Employee ranges shown are reported bands, for context.

Verification

Every listed company carries at least one verified work email, 19,597 across the full list. Contacts re-verify on a rolling cadence, and each contact's own verification date is shown inside RevenueBase.

Refresh

This page's figures were pulled from the company graph on September 23, 2026. Lists refresh periodically as the graph is re-checked.

Corrections

Wrong category, wrong headquarters, or a company that has closed? Use “Report an issue” on any row. Reports go to the RevenueBase Data Team and feed back into the company graph.