Hi Edwards,
Sorry to hear no one came back to you. Another option is to contact the local Embassy, who may shed some light on your situation and clarify the requirement for you.
If you are unable to get full confirmation before leaving, look at the testing centers near where you will be located in Spain to be prepared in case you require to test, and while in transit, ask at airports and customs for advice on the ground as well as with your airline.
Regarding entering Spain via the UK, you can find all the information
here. The UK and EU rules for entering Spain seem the same as you only need proof of vaccination at the minute.
I wish you all the best, a great holiday, and a smooth return home.