Due to the Covid-19 pandemic, entry requirements into Spain, and return requirements for those coming back to the UK may be different to those which you have encountered previously.
For up to date entry requirements for arriving in Spain, please visit this website;
Foreign Travel Advice
For up to date requirements relating to your return back to the UK, then visit the page on the FCO website here; https://www.gov.uk/provide-journey-contact-details-before-travel-uk .
This information can change on a regular basis, so it would be prudent for you to check the Spain and UK entry requirements on a regular basis, between now and the event.
Thanks to Sutton Travel for this information.