I’ve just done a round trip to Barcelona, and got the check in email from BA that is being discussed here. It caused me a bit of confusion, but I chose to ignore it. I uploaded nothing, and was still able to check in online, and when I got to Heathrow, nothing was asked for other than my passport & how many bags I had with me.

On arrival into Barcelona, I had to show the SPT QR code that I’d obtained which is due to my vaccination status being classed as unvaccinated by the Spanish. I did take electronic proof of a negative Covid test (again I need this because of how Spain class me) in case I was asked, and I wasn’t.

In terms of masks - flight out was an Iberia aircraft. We were told we needed masks for the flight, the crew were handing them out when we go on. Masks soon came off when those of us in club were eating and drinking. Flight back was BA aircraft, the boarding gate person told everyone to put masks on - got down the jet bridge and the crew said take them off if you wish.

In Spain itself, whilst it’s claimed it’s mandatory, I would estimate maybe 10% of people were wearing masks on the metro/train system - the rest, including the police at the stations weren’t.

Hope that helps somewhat.

PS - back to the BA email, I also found a few twitter threads on it, where people did exactly what I did (ignored it) and all was fine.