The public record, at one address
Every active company registered at one postcode, read live from Companies House in about a second, with what each one says it does. Then read their own filed accounts and rank the book by who has employees. No account, no permission from anybody, and nothing handed over.
A full postcode, because the register searches on one and a partial one returns a different town. Try EC1R 0NE, which is a real practice's registered office and the one this was built against.
Companies use their accountant's address as their registered office, which is why a book like this exists at all. More than one building can share a postcode, and a residents' management company registered care of a property agent is nobody's accountancy client. The book says which buildings it found and which rows it could not place, rather than putting them all in one pile.
The companies, their numbers, their status and what they filed as doing, grouped by the building each one writes. That is four requests to the register and about a second.
A second pass reads each company's own filed accounts: employees, fixed assets and net assets, each carrying the date it describes and the day it reached the register. That is a job rather than a page, and it carries on when nobody is watching.
The register can say who has an exposure. It cannot say who lacks cover: a filed balance sheet has never had a line for insurance in it, so an absence here is an absence of a field and not evidence about anybody. A company whose accounts could not be read is counted and named rather than counted as nothing.