As from tomorrow...
To enter Spain from anywhere outside the EU and considered a risk, which obviously includes the UK, now must prove they have a full (double) vaccination.
There are some countries which are considered less risk. These are Argentina, Australia, Bahrain, Canada, Chile, China, Colombia, Indonesia, Jordan, Kuwait, New Zealand, Peru, Qatar, Rwanda, Saudi Arabia, South Korea, United Arab Emirates, Uruguay, Hong Kong, Macao and Taiwan. Individuals flying from those countries to Spain do have to fill out a health control form but do not have to show any proof of vaccination, recovery or a negative test.
Visitirs from high risk level countries (mostly African ones) so so have to isolate fur 10 days in arrival as well as all the usual tests.
Countries within the EU can enter Spain without a vaccination certificate as long as they are on the "green list" - currently that list includes no EU countries.