Overview
If you build anything that touches UK company financials, you will probably assume that structured accounts data is easy to get. After all, iXBRL has been mandatory for over a decade, listed companies are heavily regulated, and XBRL is routinely described as core market infrastructure.
That assumption breaks down very quickly once you move beyond the largest companies. In practice, there is currently no comprehensive, publicly accessible source of XBRL or iXBRL filings for smaller UK listed companies. For AIM in particular, Companies House is effectively unusable as a structured data source until at least April 2027. The FCA's National Storage Mechanism (NSM) remains the only viable route — largely by accident rather than by design.
This post explains why that gap exists, why it surprises so many developers and market participants, and what actually changes in 2027.
The paradox: mandatory XBRL, nowhere to get it
At first glance, the UK appears to be a model market for structured financial reporting:
- All UK companies have been required to prepare accounts in iXBRL for HMRC since 2010.
- Listed companies operate under extensive disclosure and transparency rules.
- XBRL is routinely cited by regulators as foundational market infrastructure.
Yet developers quickly hit an uncomfortable reality:
There is no reliable, bulk, machine‑readable source of XBRL for smaller listed companies.
This is not a tooling issue or an API gap. It is how the system has been designed.
Why Companies House does not work for AIM
Companies House is the most obvious place to look — and the most misleading.
Key facts:
- Filing accounts in iXBRL at Companies House is voluntary until April 2027.
- The vast majority of AIM companies still file accounts on paper or as PDFs.
- Companies House bulk data only includes electronically filed accounts.
The result is stark:
- Roughly 97% of AIM companies do not file usable electronic accounts at Companies House.
- Fewer than 1% have iXBRL available via the Companies House API or bulk data.
In practical terms, Companies House cannot currently be used as a structured financial data source for AIM companies.
But the iXBRL already exists (HMRC)
This is where most reasonable assumptions break down.
Every UK‑registered AIM company already produces iXBRL accounts — because HMRC requires it.
However:
- Those filings are submitted as part of corporation tax returns.
- They are treated as tax data, not public market data.
- They are not accessible, reusable, or redistributable.
So the UK ends up in an odd position:
- Structured data exists.
- The state holds it.
- Markets and developers cannot access it.
Public filings, meanwhile, remain dominated by unstructured PDFs.
The FCA NSM: the accidental monopoly
For listed companies, the FCA's National Storage Mechanism has become the only practical source of iXBRL.
This was never its original purpose, but today:
- The NSM is the only place where AIM company iXBRL can be found at scale.
- Tagging quality is inconsistent.
- Validation standards are uneven.
- Downstream reuse is fragile.
Nevertheless, if you want structured financials for smaller UK listed companies in 2025, the NSM is the only realistic option.
Why this surprises so many people
The confusion usually comes from conflating three separate systems:
- HMRC — universal iXBRL, but private.
- Companies House — public, but historically paper‑friendly.
- FCA / NSM — public and structured, but only for listed disclosures.
From the outside, it feels like one coherent reporting pipeline. In reality, it is three loosely connected regimes with different incentives and timelines.
April 2027: the inflection point
The Economic Crime and Corporate Transparency Act changes this landscape.
From April 2027:
- Paper and WebFiling routes at Companies House close.
- Accounts must be filed using approved software.
- iXBRL becomes mandatory.
- Structured data should finally appear in Companies House bulk datasets.
Only at that point does Companies House begin to resemble the central structured registry many assume it already is.
Until then, expectations need to be set accordingly.
What this means for developers and platforms
If you are building products that rely on UK company financials today:
- Do not assume Companies House provides structured data for listed companies.
- Expect AIM coverage to be effectively zero outside the NSM.
- Plan explicitly around NSM ingestion and normalisation.
- Treat 2027 as a genuine reset point, not a minor upgrade.
Why this matters
Structured financial data is not a nice‑to‑have. It underpins:
- Automated analysis
- Comparability
- Market transparency
- Retail investor access
The fact that it effectively disappears for smaller listed companies is not accidental — but it is increasingly untenable.
Understanding this gap is the first step towards fixing it.
This post reflects hands‑on investigation carried out while building Ticker's financial data infrastructure. If you're wrestling with the same issues, you're not missing anything — the system really does work like this.