IN SUMMARY
if you come to Spain as a tourist, ONLY fully vaccinated with proof (as of 12 years of age).
If you return to Spain as a registered resident, EITHER fully vaccinated OR with negative test.
FUTHERMORE
Should the UK be added to the list of high-risk countries, then just fully vaccinated will not be sufficient, a negative test will be required then as well.
TESTING & VACCINATION REQUIREMENTS FOR ARRIVALS TO SPAIN FROM THE UK
The Spanish government requires all arrivals to Spain from the UK (excluding children under the age of 12 years old), to present on entry, proof of being fully vaccinated at least 14 days prior to arrival in Spain (date(s) of vaccination must be specified), with a vaccine authorised by the European Medicines Agency or by the World Health Organisation.
All passengers (including children under 12 years old) travelling by air or sea must complete and sign an online Health Control Form no more than 48 hours prior to travel.
Spain will accept the UK’s COVID-19 vaccination record. If you are travelling with a printed PDF proof of vaccination status, it must date from 1 November to ensure that the certificate can be scanned successfully.
CERTIFICATE OF RECOVERY NOT ACCEPTED
Certificates of recovery - a medical document certifying that you have recovered from COVID-19 in the last 6 months prior to travel - is not currently accepted for arrivals from the UK. See the Spanish Ministry of Health ‘Travel and COVID-19’ page for details.
https://www.spth.gob.es/faq?tab=2
Documents can be in either English, Spanish, French or German and in paper or electronic format. All documents must specify the name and surname of the passenger. See ‘Entry requirements for entry in Spain from third countries’ section ‘k.’ on the Spanish Ministry of Health ‘Travel and COVID-19’ page for details regarding proof of vaccination that can be presented when travelling to Spain from the UK.
Further rules may apply if you have travelled to a country on Spain’s list of ‘risk’ countries in the 14 days prior to travel to Spain.