Most of the 90s don't have buckeyes fitted any more, as I recall (think it was only the GA ones which still had them). As regards Mk4 sets, there are standard buffers and coupling hooks on the outer ends of the set - i.e. the TSOE loco end and the DVT nose end. Internally, they use fixed-head buckeyes (so if internal vehicles from the set are moved on their own, barriers are needed as with HST trailers). So for a set which could be coupled to non-buckeye locos at both ends, it would need either a TSOE or DVT at each end. A DVT would be dead space, but without it there would be no brake vehicle or guard's accommodation. The physical wiring between vehicles on Mk4s and earlier coaches is different, so it wouldn't be possible to mix and match - e.g. mk4s with a mk2 brake. Not sure how the door control circuits work either - i.e. whether they would work without a DVT.
But perhaps more to the point, LSL owns the mk3s as part of their charter fleet. They are hardly going to rent something else! I'm also not sure whether mk4s are cleared north of Skipton.
Not entirely sure what the PRM rules are for charter fleets, but I'm sure they are less restrictive as these are not ordinary public service trains. Clearly slam doors are fine, given that all charter operators have only slam door stock, and LSL are expanding their fleet at the moment (mk3s, HST).
Incidentally, anyone seen any mention of whether they are going to be insisting on masks, like service trains? I'm local to the area so might well have a trip out, but if masks are insisted on then forget it!
