Those cloud hosts like AWS have infrastructure in the UK and a UK bank's data should already be there for performance reasons (see https://docs.aws.amazon.com/global-infrastructure/latest/reg...).

I can't promise that amazon has taken into account the possibility of not being able to phone home to the US, but I would hope an undersea cable cut is something they'd have already given serious consideration to. Cuts do happen every so often. Scandinavia in particular has seen several disruptions because of cable cuts.

Either way, it's a bit of leap to go from "We need to jam other countries satellites to protect your banking app" to "but what if the UK lost every satellite, and every undersea cable was cut, and all the local data centers and internet infrastructure in the UK stopped working!"