See Companies Act 2006, sec854 - sec859. The Companies House website has more detailed guidance.
Every year a company is required to make an annual return to Companies House. This is a form setting out the registered details of the company: its name, number, registered office address, business activities, details of directors, secretary and shareholders, etc. The form will be sent out by Companies House and must be signed and returned with the filing fee, which is £30 if done on paper and £15 if done by electronic filing.
The purposes of the annual return are: