Exchanging Your Spanish Residency Certificate for the TIE for British UK Nationals
The TIE, Tarjeta de Indentidad de Extranjero, is the Spanish identification card for citizens from third countries (non-EU) who reside in Spain. Since July 2020 British UK nationals moving to Spain have also had to apply for this card, as the UK is no longer in the EU.
If you already have a Spanish residency certificate, you do not have to apply for the TIE, but you can voluntarily exchange your certificate for the card.
Both the Spanish and UK Government websites and their Consulate pages confirm that the green residency certificate, A4 and credit card sized, remain valid for UK nationals and prove the holders residency and retained rights under the withdrawal agreement, having settled in Spain before the UK left the EU. However considering the issues some have encountered during the recent Covid19 travel restrictions, e.g. multiple incidences of authorities and airlines not understanding the rules and denying certificate holders entry to Spain, it is advisable to get the TIE.
Aside from this the card is a full bio-metric national ID card, therefore so much more useful than the paper certificate. The card also has the words ARTICULO 50 TUE, a reference to note that the holder was resident in Spain before the end of the transition period and has retained rights.