I thought FGW weren't getting many 365s, certainly not 21 of them! I take it this means the additional 6 387s have been ordered? Sorry if this has been discussed already, I really am not up to date with all the movements!
As for the 317s and 321s, it would make sense to send some of them at least. That would at least allow TPX to give Northern their 156s back, once the 321s have been suitably refurbished of course. Mind you, I suppose TPX will be getting their new fleet by then!
Sure LM would happily snap up some of either class, since they need more EMUs in the next couple of years for Bromsgrove and Rugeley as it is plus more still to allow for growth etc.
Too early for ATW and the Cardiff Valleys, although if they made the Swanline services exclusively between Swansea and Newport only they could use EMUs and use the DMUs on said services to relief overcrowding elsewhere.
FGW's Cardiff to Taunton services should be able to go over to EMUs by then too shouldn't they? If that's already part of the plan with 365s etc then my apologies, as I mention above I'm a bit lost in what's happening when and where!