If you have a test 72 hours before departure there is no guarantee you remain virus free in the following 72 hours. The temperature check is very accurate and hopefully a vaccine will arrive soon.
If Spain/UK want to revive their tourism industry the cost of tests must be free or at least a reasonable cost. Currently a test is about £120 and to avoid UK quarantine on return to the UK a test is required both ways. It would cost a family of 4 nearly £1000, a cost I am sure they will not pay.