Companies House
What it publishes
Every UK company's filings: accounts, confirmation statements, officers, persons with significant control, charges, disqualified directors, and the Register of Overseas Entities.
How often
Real time; free bulk snapshots monthly. It has a documented data API.
Checking its records
Filings are statements made by the company to the registrar under statute. Cite the filing's document link and date. API needs a free key.
Nothing on this site cites Companies House yet.