Well - being practical about it, you need 2 power cars not one and almost have the set in fixed formation - which as you say isn't what most heritage railways want.
There are are half a dozen heritage railways which *could* possibly justify an HST given the length of their line and possible storage - Severn Valley, Nene Valley, Great Central, East Lancs, West Somerset and at a push the Mid-Norfolk. Most other lines are too short or would lack the space to store one.
I cannot see Heritage Railways taking many other than the existing HST group, possibly charter companies might take a few the prospect of train that can go to most places and not have to be looped out of the way on the fast routes might have some merit, but on the other hand they may just prefer Anglia MK3's.