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. Traditionally this was 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. An annual return can still be filed on paper, but most companies now file their annual return electronically using the Companies House webfiling service. The filing fee is £40 if done on paper and £13 if done by electronic filing.
The purposes of the annual return are: