Because all of the best studios are in the UKReally Disney? Recording this in London? Why couldn't you record it here in America. There are tons of musicians and orchestras looking for work like this. There are tons of studios built here that you could of used. It's disappointing. There's no reason for it.
If you look at film soundtracks, an awful lot are recorded in the UK. I think Disney already have a good relationship with AIR as a few MCU films as well as Cinderella soundtracks were done there in recent past. Maybe they tapped it onto the end of another films session?
If it's any consolation, looks like the vocals were recorded in the States.