One rumour is one of the bidders proposed replacing every train in the Anglia fleet. We don't know if that's true or not - one consistent DMU fleet and releasing DMUs for bidders of other franchises like LM and EMT had logic to it, one consistent EMU fleet and having 94 x 321s off-lease doesn't. However, if someone has submitted a bid which includes replacing 321s it doesn't sound illogical that they want to reconsider and state Brexit for wanting to reconsider.
Greater Anglia needs a clear out, it's just a mess in terms of stock. What possessed National Express to order Electrostars instead of Class 350 Desiro stock, for example ?
You would need to accept some stock being scrapped - probably all of the Class 317 units, but we're at that stage already, aren't we. We've certainly accepted they're expendable. The Class 321 stock, as you say, is an issue.
The only thing I can think of is for the 94 x Class 321 units to re-tractioned and re-geared to 75mph, working in pairs for the South Wales Valleys, 8 x 20m platforms rather than the proposed 6 x 23m might be do-able and you get something fairly similar to the Glasgow Class 320 units (albeit a coach longer per unit) which would work well there.
I'd assume you would send the Class 360 units to London Midland, the 21 Class 360 units from Greater Anglia plus the 5 from Heathrow Connect/Express could either provide additional capacity, or maybe even release the dual voltage Class 350/1 fleet for South West Trains, adding in some additional flexibility if there's to be any progress on schemes such as the North Downs electrification at 25kV AC.
The remainder of the fleet at Greater Anglia would presumably then be backfilled by Bombardier EMU stock, a mixture of 5 x 23m 125mph stock and 4 x 20m 100mph/110mph stock, all of which is compatible with the existing Class 379 units.
DMU stock - do you dump a new CAF fleet at Greater Anglia, or do you infill the missing electrification and expand the EMU fleet instead, or a combination of both so as infill electrification makes the new CAF stock redundant, it can transfer to Northern, allowing older stock to be replaced ?
It's all terribly expensive though, very dependent on works capacity, and ultimately needs decisions on electrification which aren't going to come soon enough to make it all fall into place, even if someone was that ambitious.