If the UK leaves the EU without a deal, any UK national residing in Spain before the date the UK leaves will be considered legally resident for a period of 21 months, irrespective of whether they currently hold a residency document.
https://www.gov.uk/guidance/living-in-spain
https://www.lamoncloa.gob.es/lang/en/brexit/howtoprepare/Paginas/citizens.aspx
Copyright © 2021 OPM Response Ltd. Tollgate Business Park, Colchester, Essex, CO3 8AB, United Kingdom