There are more than enough 319s to run both the Thames Valley services and the Lancashire Triangle (based on current plans/ operating levels).
However the "Thameslink will have some spare 319s" + "newly electrified lines will need EMUs" = "319s go to Thames Valley/ Lancashire Triangle" equation isn't guaranteed. 319s could go elsewhere (with other older EMUs moving west/north instead)
What interests me is if Southern take over the existing TL Inners (St Albans to Luton via Sutton services and run it as proposed between Blackfriars and Blackfriars via Sutton instead.
If this is the case then would we NOT see the NXEMUs on the loop as they wouldn't actually be part of the Thameslink route as such and so any rolling stock would to be Southern's responsibility and not whoever succeeds FCC.
This being the case, they might decide to keep a small fleet of 319s for the Suttons - after all the 375/6s operated by South Eastern have both AC and DC equipment and they're only used on 3rd rail routes.
As to the GN, I'm sure the basic understanding is for the 365s to stay or be replaced by 377s with the 321s and 317s moving elsewhere.
Now if 319s don't end up going to work the Lancashire Triangle, then it probably would be better if Northern or their successor asked for the 321s from FCC to bolster their own 321 fleet especially as brand new EMUs would be a rarity.