....we could well be moving towards a time where any meaningful training is simply impractical as EGTK becomes another Biggin Hill
I seriously doubt it. Whatever the current management choose to call Oxford (Kidlington) airport, it is still well over an hour by road from central London and that's too far for most bizjet owners. It's unlikely that Oxford will increase its jet movements much over today's figures and will always need the GA element to keep it viable. Quite how it will achieve this when CAE move out and Airways go under will be interesting to see.
Incidentally, as well as the already quoted locations, OATS also had a presence at Dundee for a brief period to cover one specific contract. Carlisle was a duplicate operation, not much, if any smaller than Oxford. Cadets were allocated to one base or the other and completed the whole of their course there.