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.

This article is about how that happens, why it's worse than most people realise, and how to build a genuinely complete picture of your addressable market using UK public data.

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problem: they rely on self-reported or scraped data that is incomplete, inconsistently categorised, and often years out of date.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. Their headcount is wrong, their industry classification is wrong, or their page hasn't been updated since they launched. 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. And the coverage gap is not random: it systematically undercounts companies that are growing quickly but haven't yet reached the size where they appear in the databases providers prioritise.

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 includes the company name and registered address, the names and appointment dates of every current and former director, annual accounts showing financial position (and for many companies, turnover), persons with significant control, filing history, and the company's Standard Industrial Classification (SIC) code - its stated industry category.

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. And because it's a legal obligation to maintain the register, the data is more reliable than self-reported profiles.

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. And the SIC code system - the primary way of categorising companies by what they do - has some serious problems for anyone trying to use it to find sales prospects.

Using SIC codes to map a market - and why they fall short

SIC codes are the UK government's taxonomy for classifying business activity. They were designed in the mid-twentieth century and updated most recently in 2007. Every company on the register is required to assign itself at least one SIC code.

In theory, this should make it straightforward to find all the companies in a given sector. Want to find IT consultancies? Filter by SIC code 62020. Marketing agencies? Try 73110. Management consultancies? 70229.

In practice, it's considerably messier than that.

The first problem is breadth. The SIC taxonomy has around 730 codes covering the entire UK economy. Many of those codes are very broad. "Other business support service activities" (82990) covers everything from call centres to mystery shoppers to conference organisers. "Other professional, scientific and technical activities" (74909) is similarly wide. Companies in these codes may have nothing in common beyond the fact that their accountant couldn't find a better fit.

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. A digital marketing agency might file under 73110 (advertising agencies), 74100 (specialised design activities), 62020 (IT consultancy), or 70229 (management consultancy), depending on how the person registering the company described the business at the time.

The third problem - the one that hits horizontal sellers hardest - is that your target market is by definition spread across many codes. A product that sells to any growing professional services firm might have ideal customers classified under a dozen different SIC codes. If you filter by the two or three most obvious ones, you miss the rest.

Research on this consistently shows that for horizontal B2B products, SIC-based filtering misses between 30% and 60% of the actual addressable market. That's not a rounding error. That's a significant portion of your potential pipeline that never appears in your prospecting tools.

The gap between SIC classification and what companies actually do

The underlying issue is that SIC codes describe a company's primary economic activity as it was classified at a point in time. They don't describe what the company actually does today, who runs it, whether it's growing, or whether it has the profile of your ideal customer.

A company that was founded as a software development shop (62012) and has since evolved into a product-led SaaS business is still classified as 62012. A professional services firm that started as a management consultancy (70229) but now does a mix of consulting, training, and software implementation is still 70229. A fast-growing e-commerce enablement business might be classified under retail (47910) because that's where the incorporation agent put it.

None of these companies are miscategorised in a way that matters for regulatory purposes. But for sales prospecting, the classification creates an invisible wall: if you're filtering by SIC code, these companies are invisible to you unless you happen to include the right code in your search.

The gap between SIC classification and commercial reality is why natural language search - describing what you're looking for rather than selecting categories - consistently finds more of your real market than filter-based approaches.

Step-by-step: building a complete account universe

Here's how to approach TAM mapping for a UK B2B product in a way that captures more of the market than a SIC filter alone.

Step 1: Define your ICP in plain language. Before touching any tool or filter, write a description of your ideal customer in the same terms you'd use to explain it to someone who doesn't work in sales. "Growing professional services firms with 10 to 50 employees, at least one director-level hire in the last 18 months, and between £1M and £8M in annual revenue." This description will be more useful than a list of SIC codes.

Step 2: Identify the SIC codes that are most likely to contain your ideal customers. Use the reference table below as a starting point. These are not the only codes your customers will appear under, but they're the ones where the highest density of relevant companies is likely to be. This gives you a baseline list.

Step 3: Layer in web and job posting data to catch companies the SIC filter misses. A company posting roles that match your buyer profile - "Head of Operations", "Revenue Operations Manager", "Client Services Director" - is telling you more about what they actually do and where they are in their growth journey than any SIC code. These companies may not appear in your SIC-filtered list.

Step 4: Validate with financial trajectory. For any company in your candidate list, check whether the financial profile matches. A company that looks right by sector and headcount but has been flat for four years is a different proposition from one showing consistent growth. Companies House filed accounts give you this for free.

Step 5: Maintain the list as a live view, not a one-time export. New companies are incorporated every day. Existing companies cross your revenue threshold. Leadership changes. The market moves. A TAM that you mapped six months ago is already out of date. The most effective teams treat their account universe as something that gets refreshed continuously, not a spreadsheet they pulled once.

SIC code reference table for UK B2B prospecting

This table maps common UK B2B sales targets to their most likely SIC codes. It is not exhaustive - as noted above, companies are often miscategorised - but it is a useful starting point for building a candidate list.

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. The companies you find in a SIC-filtered search should always be validated against the actual company profile before entering your prospecting workflow.

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 language 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. When a new company crosses into your ICP (growing into your revenue band, making a relevant hire, appointing a new director), it surfaces automatically.

For teams trying to scope their TAM before a fundraise, entering a new segment, or simply trying to understand whether their ICP is too narrow, this gives a much more complete picture than any filter-based approach.

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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 to map a UK B2B target market 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 because companies self-assign codes inconsistently and your market is likely spread across many codes. Combining Companies House data with job posting signals and web presence data captures significantly more of your real addressable market.

What is a SIC code and why does it matter for B2B prospecting?

A SIC (Standard Industrial Classification) code is a number assigned to every UK registered company that describes its primary business activity. They are the most widely used filter in UK prospecting tools and company databases. 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 to distinguish meaningfully between companies within the same category. For horizontal B2B products especially, relying on SIC codes means missing a significant proportion of your target market.

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. The vast majority are small businesses with fewer than 10 employees. The UK's company register is one of the most comprehensive public business databases in the world, covering every incorporated entity regardless of size or sector.

What information is publicly available on UK companies?

For every UK registered company, Companies House holds the company name and address, director names and appointment dates, annual accounts (showing net assets and sometimes turnover), persons with significant control, and SIC code classification. This data is free to access. The limitation for sales teams is using it at scale - searching across millions of companies and keeping the results current - which requires either significant manual effort or dedicated tooling.

How do I build a TAM for a B2B product that sells across multiple sectors?

For horizontal products, the most reliable approach is to define your TAM by company characteristics (revenue band, headcount range, growth trajectory, director profile) rather than by sector. Sector-based TAM analysis will always undercount your market when your best customers are spread across many industries. Start with a plain-language description of your ideal customer, use the Companies House dataset to find companies matching those financial and structural characteristics, and layer in job posting signals to identify companies whose hiring patterns match your buyer profile.