What is the likelihood of somewhere like the NYMR buying a shortened set when they’re eventually disposed of, to operate off-peak diesel services through to Whitby? Somewhere near 0%?
It's quite an undertaking, preserving a whole train, even a shortened one, two locos plus carriages, in one go: Let alone finding sufficient space to stable and maintain it - even a 2+4 HST formation (and you wouldn't want to go any shorter if you want to ensure decent passenger receipts, presumably) isn't much shorter than a seven carriage mark 1 rake. Quite fuel thirsty things, too: Less so in the 25 - 35mph bracket, no doubt, and I suppose you could just keep one power car running, but if you're going to do that then there's little benefit (other than dispensing with the need to run round) over sourcing a traditional diesel loco and a short carriage rake from the existing fleet.
Never say never, of course, and it's possible that one or more of the more established lines might cherry pick power cars and carriages as money allows to form up a complete rake, but I'd still be looking mainly at the 125 Group to fulfil our future HST requirements in preservation. I can't see many preserved lines being able to support a year round need for an HST set of their own - and bear in mind that the air conditioned mark 3s could suffer issues with moisture over conventionally ventilated stock if laid up for any period of time.