Why are the 387/3s heading for southern? If the 387/2s are to return, what will be in its place.
The GN operation appears to have been planned around 35 387s. Recently that has been 29 387/1s and 6 387/2s. There is a suggestion that the 6 387/2s stay where they are and the 6 387/3s go to Southern.
Gatwick Express isn't running 4tph from Victoria (eg not the Victoria to Gatwick shuttles or indeed 4tph to Brighton in the peak) so it doesn't need the six 387/2s currently operating on the GN route. However, those 387s could have been operating on the east coastway from Brighton to Hastings.
Will GWR receive 379s and in turn cascading the 387s to Southern?
That doesn't appear to be clear at all. There is a suggestion that the 379s will move from their current storage locations in September.
I note that other sidings which have been used to store withdrawn trains may be clear by September with 317s appearing to be very close to being finished.
People appear to be stating that Southern has an overcrowding problem but it is very difficult to get a clear picture of which trains it is and at what times other than that five car trains via Tulse Hill may have been a step too far. There are some timetable changes in September which change frequencies on some of the routes but not this one.