Proving your Residence Status when entering Spain
"The Spanish government has formally confirmed to the UK government that UK nationals who were legally residing in Spain before 1 January 2021, and as such are beneficiaries of the Withdrawal Agreement, can use any of the following documents to prove their residence status when entering Spain:
Residence card issued under Article 18.4 of the Withdrawal Agreement (the TIE – Tarjeta de Identidad de Extranjero)
Temporary or permanent EU residence certificate (Certificado de Registro de Ciudadanos de la Unión)
Receipt of application for the TIE (Resguardo de presentación de la solicitud de la tarjeta de residencia)
Confirmation of the positive outcome of your residence application (Resolución favorable por la que se concede la tarjeta de residencia)
In the absence of any of the above, other documents that credibly evidence your legal residence in Spain before 1 January 2021, such as a padrón certificate (issued by your town hall), a work contract, a rental contract, or proof of property purchase may be presented. However, due to current travel restrictions, be aware that you may be questioned on arrival by Spanish border authorities to ensure you meet the legal entry requirements. Spanish border authorities will only grant entry if they are satisfied that you are returning to Spain to your habitual place of residence or can demonstrate an essential reason to enter (see ‘Permitted reasons to enter Spain’), and reserve the right to deny passage.
The above list of documentation has also been communicated to airlines and other operators.
You must also show a valid passport when you travel and comply with the other documentation requirements, as set out in this travel advice."