If you’ve had two vaccines and a booster you are fine.
If you’ve had two vaccines and the last of them was less than 270 days ago then you will be fine.
If you are not fully vaccinated and have recovered from CoVid in the last 6 months you can get a proof of recovery record.
Otherwise, you are into proving negative tests and filling in the SpTH form in the 72 hours before you go, which gets more involved.
UK Govt info here:
FCDO travel advice for Spain. Includes safety and security, insurance, entry requirements and legal differences.
www.gov.uk