GN - it’s not exactly likely 6 units would go anywhere else, hasn’t it been a fairly regular recent subject of discussion, mainly in the 365 withdrawal context? GN are already borrowing 387s from GatEx, and I think it was assumed they’d be replaced by the C2C when released.
GWR is clearly temporary, what would they actually need 6 extras for in the medium term?
What happens with GN will depend very much on what GTR choose to do overall with their fleet.
If the GN side sticks with 387s, they you could keep the 29x387/1s, add the 6x c2c units to make 35, and keep a further 13x 387/2s and that essentially matches what existed with the 365s. The 387/2s could then be replaced with 377/5s off Southeastern.
However it’s all very speculative, as there are other variables. Firstly whether peak passenger levels recover or whether a longer term reduction in stock is viable. But secondly what GTR choose to do with the 313 and 455 fleets - if anything.
Even taking into account a potential reduction in numbers, I don’t believe there are enough potential Electrostars to do away with the 313, 365 *and* 455 fleets, and this includes adding the 379s to the mix, at least not without quite a reduction in peak capacity.
So the answer is at the moment it’s too early to make firm guesses as to what might happen.
Personally I’d return the 365s to GN, bring the 379s to GN to replace the 387/1s, then throw every remaining Electrostar at Southern to replace the 313 and 455 fleets, but that’s one of numerous potential permutations, and of course I’m slightly biased as I like the 365s!