Sorry, I'm going to spoil everyone's fun and pour some ice cold water over this idea. What XC needs is more capacity. In the next two years more than enough suitable stock will be released by Avanti - completely compatible, and EMR - pretty damn close. It's the ideal home for that stock - most other routes won't benefit from the 125mph capability, and if it goes anywhere else will probably be an OTT costly solution. So XC has to be the logical home for as much of it as is needed.
The idea of bi-mode stock is great until you remember the additional costs. For the GW and EC routes the diesel performance is a step down from the HST (and presumably 220). The DfT found itself in a bit of a pickle with the MML, in that having cancelled electrification north of Kettering, that would mean increases in journey times with the same 800 spec, which would have caused an uproar, as it promised that cancellation would have no impact on passengers. So it's had to overspecify the diesel capability (=more cost) to ensure that didn't happen.
The same would be needed on XC, given the high proportion of diesel working. So the benefit on the electrified sections will be diminished by lugging all those diesel engines and fuel around, and on diesel, you'll have a slight inefficiency against pure diesel with weight of the ac kit, but obviously for a much greater proportion of the journey than the majority of routes that the bi-modes are currently expected to work.
So given there's an obvious solution staring the DfT in the face, why would it spend a load of money building a fleet of units and having one if not two fleets of units with at least a decade of life left in them with no appropriate home to go to? (The only possible answer would be to admit that the interior of the voyagers is pretty poor and justify replacement on those grounds, but it would be a first for the DfT to consider passenger comfort and ambience important, so I wouldn't hold my breath on that front.)