What do I need to fly to Spain from the uk then back to the uk

Hi I need help please I would like to come to Spain for a few days to check my house the last time I was out in Spain was nearly 2 years ago and I would like to come out ASAP 

I have had my 2 jags and have the Nhs letter confirming this  do I need a test before I go and one before I come back my problem is I only intended being there for 3 days 

Will the Nhs Covid test be ok to use or have I got to get something else Sorry I have heard so many different things from different people some are saying as long as I have the Nhs letter I should be ok 

Thanks and I look forward to your help

Paul