Isn't the main issue with going back to the previous timetable the fact that it was a national change, and therefore any local changes (single TOC revisions) would/could have a knock-on on the wider scale?
As I see it, going back introduces two issues:
1) Rolling stock and train crew diagrams would need to change. The former probably wouldn't be a problem except for a short transition period, except if there's a mismatch in stock for example electric vice diesel, but the latter means giving notice of changes, which requires quite a longer time lag, and summer holiday time would be the worst possible time to do it. In some cases it might be very difficult logistically if there have been changes to driver establishments at depots to fit around the new timetable.
2) If you only revert one area then sooner or later there are going to be timetable clashes at the margins. The oft-quoted LTS reversion involved a virtually self-contained network with only a handful of driver depots. It could be possible to revert certain areas and then leave it to the signalers and train crew supervisors to sort out on the day, however this could cause even more massive chaos over a wider area.
Having said that, I do think they could revert the Great Northern side of things if desparate. LNER timings are barely changed, so they could also revert at the London end without too much difficulty. This leaves the two open-access operators which shouldn't be that much of a problem (although there might be commercial issues), plus Cambridge-Kings Lynn and the Peterborough area, none of which should be massive showstoppers with a bit of planning, assuming there's resources available to do it of course. Then just the small matter of deciding what you do with 3tph south of the river, many of which haven't been running anyway, and where there are other alternative services available in many cases.