[DTV]

This is assuming the availability of temporary sets as the only limiting factor in the rollout. We don't know how the logistics work. Maybe there are two crews traveling around the regions building the new sets one after the other. In that case, you simply wouldn't need more than two temp sets because only two could ever be built at any one time.

Yes, I suspect this is part of the case too - given the uniformity of the sets, I imagine they are being constructed, and to at least some extent installed, by the same teams. That said, I doubt the full 15 weeks of the Nottingham upgrade were spent on installation - there will have been at least a few days of training/rehearsals and I suspect that they'll have taken advantage of the studio being completely empty for the first time years to do other technical work (the last Southampton refit coincided with a new studio floor being put in).