The latest information from the UK Foreign & Commonwealth Office, with clear guidance.
Residency documents and status
You and your family may need to apply for a residence status to confirm that you were already resident in the EU country you live in before 31 December 2020. You will have until at least 30 June 2021 to do this.
The EU country where you live may set up a system for applying for a residence status. The application should be short, simple and either free of charge, or cost no more than applying for a similar document, for example, a national identity card or passport.
You will have until at least 30 June 2021 to submit your application
Source UK FCO 30/1/20
Full details here Living in Europe