[bbctvtechop]

They have many DR plans for NBH, with different studios in different fire zones, and also Millbank which IIRC was used for the 6 once.

I suppose it's sensible to also have Salford and others if they can switch to feeding to the whole network on their own.

Correct, and the "London is not available" DR plan is for Salford to handle one stream of TV news and one stream of Radio news across all networks. Clearly this is ultimate DR planning where every BBC building in London is unavailable and the situation requires every TV channel and every radio channel to carry the same programming for days or weeks at a time. Obviously in such a scenario the country has many more problems than a BBC studio not having the right closing music available!