I was thinking the same, it does seem odd to have a whole other gallery sitting there in reserve. Maybe it's because of the complexity of the VR? Though if they're in so much trouble that they can't use their main gallery they should be happy just with a basic setup just to get them on air.
You're right in that the BBC have the advantage that they have multiple studios to relocate into (and they do that as a matter of course every day) though they are of course a very different type of output, they aren't just doing 2 hours of bulletins a day.
Could the spare control room be not really a spare? If it's set up the same the ITV London crew can go in and prep before the show.
I believe most US broadcasters have a backup (maybe not as fully featured) control room in their premises. Some networks are standardizing their control rooms so any studio can be used with any control room.