Residency Document for British Citizen

In July of 2020, General Directorate of Migration and the Directorate General of the Police of Spain have published instructions for the procedure of the issuance of the residence document for citizens of the United Kingdom in Spain following Brexit, implementing the regulations of article 18 of the Agreement on the withdrawal of the United Kingdom of Great Britain and Northern Ireland from the European Union and the European Atomic Energy Community.

The central points of the instructions:

They apply to those UK nationals, as well as members of their family, who were residents in Spain before december 31 of 2020 and continue to reside after this date.
Residents benefiting from the regulations of the withdrawal agreement do not have to apply for a new resident status in Spain. They do, however, have the right to receive a document certifying their status of residence in accordance with the withdrawal agreement. The request of this document is possible since July 2020.
Those people applying for residence after the transition period (after the 31 of december 2020) have a period of three months to present their application.
The document is valid for either 5 years (temporal document) or 10 years (permanent document), depending on the duration of previous legal residency.

What do you have to do?

If you are a british citizen holding a Residendy Certificate for EU nationals but you have not reached 5 years of legal residence in Spain, you can apply for a temporal residency document with a validity of 5 years personally at the corresponding station of the National Police (comisaría de Policía Nacional).
If you have legally resided in Spain for a period superior than 5 years, you can apply for the permanent residency, valid for 10 years. You can do this whether you have a temporary or a permanent Residency Certificate, having to verify the duration of legal residence in the first case.
If you are a british citizen without a Residency Certificate, you can request the residency document at the local immigration office, either personally, through a representative, or by electronic means. When the request is granted, you apply for the sending of your residency document personally at the corresponding police station.

If you are applying for the document as a relative of a british citizen and are the holder of a Residence Card for Relatives of European Union Citizens, you can request de residency document at the corresponding police station, the validity (either 5 or 10 years) depending on the duration of previous legal residency.
If you are not holding such a Residence Card, you apply for the residency document at the local immigration office, having to bring the supporting documentation.