After further thoughts, I think a solution for the "22x" problem would be for XC to obtain a fleet of (at least) 7 coach bi-modes to cover all routes that run partly "under the wires".
Then, for the 221 fleet, remove the existing diesel engines, and replace them with lighter, lower-power engines - just enough power for them to run at 100 mph - and much lower fuel consumption; also remove tilt mechanisms. Reduce 1st Class to no more than half a coach, then use them to replace 170s on Nottingham/Cardiff & Birmingham/Stansted services. That would provide much needed extra capacity on those routes.
The displaced 170s to be cascaded elsewhere - to increase capacity, or maybe to start replacement of older 15x units.
Retain at least some of the 220s for any non-electrified routes, but always run them in 4+4 formations.