Historically, there was a reason for the HSTs to be at confined to the fast Nottinghams (and I know that these call at Harborough, but that's only a result of one of the slows going to Corby and obviously missing Harboro'). There were periods when the failure rate of the HSTs was very high (I spent many a commute ambling along on an OPCO HST) and this was blamed on their unsuitability for use on services with comparatively frequent stops, and the solution was to confine them to workings with a limited number of calls.
Now I know this was in the days of the original type of Paxman engine, but it does seem that there was been a reluctance from the Nat Exp MML days and throughout the Stagecoach ones not to follow this practice (and I know it's different on Sundays and there are possible explanations for this).
Of course, it may be that the re-engine LNER ones are seen as not having this short-coming, but it would suggest that there will just be like-for-like swaps. And, as an aside, aren't there enough soon-to-be redundant LNER HSTs to replace the ex Grand Central ones as well, without the need for 180s, exciting as the prospect of the latter might be?