I´ve been trying to get an answer from the British Embassy, but all they sent me was wording from the Gov.UK website (which I´d already seen and referenced in my email to them!). You´ll see that it says the letter of invitation "may" be needed and it´s only "one option". These are...
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... the requirements for other Third Country Nationals; however, the UK was granted visa-free travel in the EU as part of the Brexit deal so, in theory, we shouldn´t need to do this. I think Gov.UK is covering itself because the Spanish government´s website is ambiguous too, but until someone is denied entry at the Spanish border, we won´t know for sure.
"At Spanish border control, you may need to use separate lanes from EU, EEA and Swiss citizens when queueing. Your passport may be stamped on entry and exit. You may also need to:
show a return or onward ticketshow you have enough money for your stayshow proof of accommodation for your stay, for example, a hotel booking confirmation, proof of address if visiting your own property (e.g. second home), or an invitation from your host or proof of their address if staying with a third party, friends or family. The Spanish Government has clarified that the “carta de invitation” is one of the options available to prove that you have accommodation if staying with friends or family. More information is available from the Spanish
Ministry of Interior."
https://www.gov.uk/foreign-travel-advice/spain/entry-requirements
What happens when ETIAS is introduced next year is anybody´s guess!
Kind regards,
Kim