My brother has just booked a British Airways Flight from U.K to Seville on the 3rd July at a very reasonable imo £47. Will he need to quarantine upon arrival or not?

Hola

In theory - No Spain will be open - Government has declared that 1st July Spain will be open UNLESS there is a second wave or they decide that the UK is a dangerous place and they don't want people from that country

Davexf

As an aside, does your brother know how hot will it be in Sevilla in July? Or will he go on to somewhere else in Spain?

Yes he lived in cordoba for several years so is well aware of heat, but will be heading to the coast after doing what he needs to get done in Seville.

DH and I were in Seville September two years ago. The heat was unreal! Lovely place though.

Will he be able to get back into the UK or will he be quarantined?

I stayed in Sevilla for a few months from December and even before May it was getting inferno.

Open ended and flexible to duration. If he has to quarantine when he does return to U.K in Sept/Oct its not an issue for him.

Seville airport is expanding and much less like a "cattle market" than Malaga. Before this fiasco with the covid virus, there were more tourists in Seville than I have seen before. Yes it is hot in summer, but just do what the locals do, and stay indoors during the afternoon . Of course the costas will be cooler, and don't forget the atlantic coast, which we prefer to the med.
Pete