JF
Also if the whole complex went down they also have a Westminster Studio 3.5 miles away
Isn't BBC London based out of the Peel wing? Same complex of course so if that studio has any problems it's worth having D spare. Ditto the Arabic- and Persian-language services apparently in that same wing. Especially with B gone, it probably makes sense to keep one studio spare for literally whoever might need it.
Also if the whole complex went down they also have a Westminster Studio 3.5 miles away