Would you care to suggest how a business case for spot hire could be remotely feasible? Who is going to hire them? How frequently?
Rather than discussing theoreticals, how about recounting the many real-life examples of occasions where LHCS has covered for the lack of available units in the last couple of decades. Considering DRS own the most compatible locomotives, it isn't entirely inconceivable that they could pick the Mk5's up cheap from a leasing company looking to cut its losses, and hire them out as needed and use them for various tasks, possibly passenger services, but even NR may be interested in them for test trains or even things like testing locomotives with trailing loads, route learning, whatever... And I'm still waiting for a source that DRS are no longer interested in supplying services to passenger TOC's
What Open Access Operator would be able to use the MK5A coaches and with what motive power?
Not sure that you would get enough power from a class 93 on diesel/battery power to haul the MK5A coaches on non electrified sections, but the only TOC I can think that may use the MK5A coaches is Scotrail. However, Scotrail I believe have a contract to be using the HST Inter7City fleet until 2030? If that is the case, then Scotrail would be better to wait until 2027 and look at Hitachi AT300 or CAF Civity Class 397 equivalents nearer the time, to lease brand new.
With regards to Transport for Wales, they have no need for them with using class 67 + Mk4, class 197, 230, 231, 398 & 756 trains.
One of the new ones on South Wales-London or Stirling-London routes that will need rolling stock quickly, or even GC if they can't get anything better than their current 180's. Anything could be happening behind the scenes right now, and stranger things have happened very recently than relatively modern carriages finding a new home. I have acknowledged the locomotive issue already and still think the difficulties of locomotive compatibility will be lesser in the UK, than if these carriages are run overseas, which works in the Mk5's favour.
Scotrail will probably go for 222's as the fleet size is right.
TFW are still having a whole host of issues with their DMU fleets, so MK5a's and 68's may still appear there on something like Manchester-North Wales, to supplement rather than replace the Mk4 sets.