Eileen, quick hijack if I may! (I know you are Spanish)
On a FB group I'm in, someone is screeching blue murder about sueing the Spanish govt because she was refused boarding as the airport staff said her LFT to return was too old. She said the UK govt says 72 hours is fine. The people at the airport say it has to be no more than 48. (that would be the case here in Italy) do you know if there is a discrepancy between what the UK accepts and what the Spanish airports ask for? And who would be correct in this case?
Thanks!
@Geamhradh
Sorry I've just got in from work and seen your post.
I cannot be 100% sure about the legalities of this case, but in Spain - as in the rest of the EU - antigen tests (LFT) are only valid for 48 hours. PCR tests are valid for 72 hours.
The UK accepts a 72h old LFT, but that is technically invalid on Spanish soil (much like in Italy).
The problem here is that I don't know why the ground staff at the airport had the power to declare that test result invalid, given that it is not a legal requirement in Spain to produce a Covid pass upon entry to the airport. The Spanish airport cannot 'ask' for anything, their only responsibility is to check the paperwork complies with what the destination country (here, UK) requires. The test only had to be checked for boarding purposes.
The only valid explanation would if the airline's own policy doesn't accept tests that are older than 48 hours?
I personally think she should've been boarded, because she complied with all the rules as much as we can take from the story.
Now, about the suing. Spanish airports are owned by a private company (AENA) that has nothing to do with the government, so I've no idea why this lady would want to sue the government, they don't own or have control of the airports... Those who denied her boarding are also not the government, but ground staff employed either by AENA, or the airline she was flying with. Or subcontracted companies of the airport. Surely she'd need to sue easyJet/TUI/whoever she flew with?
I digress but I'd love to comment on that post about how useless these actions she's saying she'll take are going to be 
To sum it up yes, there are often discrepancies between countries. Best thing is to go with the most restrictive ones, just to be safe. Don't do tests 71h 59min before you fly just because the UK government says you can. Leave some margin and get into the window that all countries recognise, because you don't want an idiot useless check in assistant misunderstanding the rules and refusing you boarding.