The issue of the First-owned HST's is something of a red herring. No operator with any serious future ambitions towards UK rail franchises, is going to commit commercial suicide by being deliberately obstructive to the smooth running of any franchise successor, by withdrawing the rolling stock required to operate the services; whether they own it or not. I would imagine that DfT would 'request' First to make the HST sets available, and should First refuse then watch their share price tumble when it becomes clear that their railway career in the UK is over!
Exactly, yet so many on here seem to believe that owning 6 HST sets means First must win the Great Western franchise. If they don't they'll throw a temper tantrum and take their trains away.
In case people forget, open access operator Hull Trains swapped its nice new Class 222/1 Pioneers (Meridians) for slightly older, and less reliable (based on FGW operation) Class 180 Adelantes. That was so EMT could get the extra 4 222s for their Corby services. I seem to recall that the Open Access operator was persuaded to swap the units by the DfT.
So even if First lost the franchise and took the HSTs away, would they really end up with Hull Trains? Does that operation need to switch from 5-car DMUs to 2+8 HST sets? What does First do with the Class 180s, on which it will have a leasing deal, presumably with a penalty clause if they're handed back early? As for ScotRail, apart from the Caledonian Sleepers it's an EMU and DMU operation. Why would Transport Scotland want 6 HST sets added to the ScotRail fleet? Where would ScotRail use them? Again, it'd be overkill bringing in a 2+8 HST set to replace at best 6-car DMUs.
So yes, First could pull the 6 sets, but it's not like you can just drop them somewhere else! Don't forget, when First got FGW they had to cut unit numbers (DfT specification) and were getting absolutely hammered in the press for overcrowding. Buying those 6 HST sets allowed FGW to replace multiple units on some services, to increase capacity that had been cut. The cost of buying the extra sets themselves was more than worth it for limiting the damage to their reputation.