I wonder if London might only be used for the weekend bulletins, read by the national presenter - if indeed the weekend bulletins use Studio B. Just can't see it happening otherwise, especially as they already have to deal with the weather presenter coming in for the final few minutes and now have the shot of all the regional newsreaders too. Studio B doesn't seem to have been designed in such a manner that it's easy to shift from one show to another in a split second, unlike Studio E where they often have the network newsreader at the desk and the news channel presenter on the catwalk able to take over almost immediately.
@Brekkietweets