How to find every company in your total addressable market in the UK
By Dave Curran, Co-Founder, Firmbase | March 2026 | 9 min read
Most B2B sales teams in the UK are working an incomplete version of their market. Not by a small margin. By a significant one.
The accounts they know about - the ones in their CRM, on their prospect lists, in their Sales Navigator searches - represent a fraction of the companies that could buy from them. The rest are invisible. Not because those companies don't exist, but because the tools used to find them are built on a flawed foundation.
Why most sales lists are incomplete
The most widely used method for finding B2B prospects in the UK is some combination of LinkedIn Sales Navigator, a data provider like Apollo, Cognism, or ZoomInfo, and manually adding companies you already know about.
Each of these has the same underlying problem: they rely on self-reported or scraped data that is incomplete, inconsistently categorised, and often years out of date.
LinkedIn's company data is only as good as what companies have chosen to put there. Many small and mid-sized UK businesses have minimal LinkedIn presence. The companies that are hardest to find on LinkedIn tend to be exactly the ones you want: focused, owner-managed businesses that are too busy to maintain a social media presence.
Third-party data providers aggregate from multiple sources, but their UK coverage drops significantly below the enterprise tier. Coverage of companies with fewer than 50 employees - which is most of the UK's 5.5 million registered businesses - is patchy at best.
The result is a market map with large blind spots. The accounts you can't see aren't bad fits. They're companies doing the same thing as your best customers, at the same size, in the same growth phase - they're just not in the dataset you're using to find them.
How the UK company register works
The UK has one of the most comprehensive public company databases in the world: Companies House. Every company incorporated in England, Wales, Scotland, or Northern Ireland is registered there. Over 5.5 million companies, with filings going back decades, available for free.
What Companies House holds on each company: the company name and registered address, the names and appointment dates of every current and former director, annual accounts showing financial position, persons with significant control, filing history, and the company's SIC code.
This is significantly more complete than any third-party data provider. It covers every registered UK company, not just the ones that have been scraped or opted into a database.
The limitation is that Companies House was built for regulatory compliance, not for sales prospecting. The interface is designed for looking up individual companies, not for running queries across the full dataset.
Using SIC codes to map a market - and why they fall short
SIC codes are the UK government's taxonomy for classifying business activity. Every company on the register is required to assign itself at least one SIC code.
The first problem is breadth. Many codes are very broad. "Other business support service activities" covers everything from call centres to mystery shoppers to conference organisers.
The second problem is self-assignment. Companies choose their own SIC codes, usually at incorporation, usually with guidance from whoever is registering the company rather than from someone with deep knowledge of the classification system.
The third problem - the one that hits horizontal sellers hardest - is that your target market is by definition spread across many codes. Research on this consistently shows that for horizontal B2B products, SIC-based filtering misses between 30% and 60% of the actual addressable market.
SIC code reference table for UK B2B prospecting
A few important caveats: many companies will appear under codes not listed here. Many companies in the codes listed here will not match your ICP at all. Use this table as a starting universe, not a definitive filter.
How Firmbase approaches this
Firmbase was built on the premise that SIC codes are an unreliable starting point for finding your market. Instead of asking you to select codes from a taxonomy, Firmbase lets you describe your ideal customer in plain English and searches across Companies House data, web presence, and job posting information to find UK companies that match.
The output is a continuously updated view of your addressable market - including companies that would be missed by a SIC-filtered search - ranked by ICP fit and current buying signal strength.
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Frequently asked questions
How do I find all the companies in my target market in the UK?
The most complete way is to start with Companies House data - which covers every registered UK company - and search by company profile and financial characteristics rather than by SIC code alone. SIC-based filtering typically misses 30-60% of relevant companies for horizontal products.
What is a SIC code and why does it matter for B2B prospecting?
A SIC code is a number assigned to every UK registered company that describes its primary business activity. They matter because they are the default taxonomy for finding companies by sector - but they are unreliable for sales prospecting because they are self-assigned, often out of date, and too broad.
How many companies are registered in the UK?
Over 5.5 million companies are currently registered at Companies House, with hundreds of thousands of new incorporations each year.
How do I build a TAM for a B2B product that sells across multiple sectors?
For horizontal products, define your TAM by company characteristics (revenue band, headcount range, growth trajectory, director profile) rather than by sector. Start with a plain-language description of your ideal customer and use the Companies House dataset to find companies matching those financial and structural characteristics.
About the author
Dave Curran is the co-founder of Firmbase, a UK B2B sales intelligence tool that helps sales teams find, prioritise, and reach the right accounts without needing a RevOps team to make it work.
Firmbase helps UK B2B sales teams discover their complete account universe, prioritise based on real buying signals, and reach out with genuine relevance. Start your free trial