Companies House is the UK companies registrar. Every Ltd or PLC must file a series of public documents within strict deadlines: SH01 forms within one month of any share allotment, PSC (person-with-significant-control) register updates within 14 days, annual accounts within nine months of year-end, and a confirmation statement at least once every 12 months.
For founders, the practical impact is that share-cap changes from a fundraise become public the day they're filed (price isn't disclosed but ownership shift is), and missed filings can lead to a £150 to £1,500 late-filing penalty plus director disqualification in extreme cases. UK lawyers and accountants typically handle the filings as part of a £400 to £1,500/year compliance package.