It would be a massive stretch for GN to lose the entire 387 allocation. The general feeling is there would have to be a handful left, either permanently or rotated around with the Southern fleet.
It's not entirely correct to put it in those terms.
Class 700 utilisation is nothing like it should be - for many very obvious reasons.
In respect of the Cambridge to Maidstone, the unit saving on only going to Kings Cross must be at least 6 on a half-hourly timetable, and they were procured to be used. Add in the fact you don't have the services to Littlehampton, and there's a couple of 12 car units also not used. As a result, you have to start at the base of some of the Ely trains by all accounts being 700s in the new timetable in May which will roll over to December. That's a part of the puzzle solved, as 700s just get used on lots of GN services.
One could, I think, suggest that yes there might be a couple of 387s brought up and down to Hornsey, but one assumes it is not likely.
So the gains and losses will be something like this? Assuming SN loose 40 377/1s.
GN
+ 120, - 156, Loss of 36, 9 units
+ 379/0 - 30 x 4 = 120
- 387/1 - 32 x 4 = 128
- 387/2 - 1 x 4 = 4
- 387/3 - 6 x 4 = 24
GTR
+ 156, - 160, Loss of 4, 1 unit
+ 387/1 - 32 x 4 = 128
+ 387/2 - 1 x 4 = 4
+ 387/3 - 6 x 4 = 24
- 377/1 - 40? X 4 = 160
SE
+ 160, - 158, Loss of 2 coaches
+ 377/1 - 40? X 4 = 160
- 465/9 - 25 x 4 = 100
- 466/0 - 29 x 2 = 58
I don't think there is any evidence of 40 377s to Southeastern. It sounds like it could be as little as the 15 377/2s, as it's 377/7s only (to be confirmed) on Watford trains shortly, if not already. You've got the benefit of the entire fleet of 707s in squadron service, as well as the 376s for the 10-car railway (albeit a lot of Southeastern isn't a 10 car railway, it's 8 car on the Chatham or 12 car on the Charing Cross sides, save for Woolwich Dockyard and Hayes (?)).
Don't forget, on the main line you don't have extra peak services from Blackfriars as you once did, the Cannon Street to Thanet trains aren't all 12 car, and so one has to surmise that Southeastern doesn't need it's full fleet of 465/466 units (especially as Crossrail now does a lot of heavy lifting). The Networker fleet also harks back to a time when Metro went to Gillingham, which it doesn't anymore. It went to Faversham, Tunbridge Wells - this will all change / has changed.
The current number of 465/9s and 466s is only 158 coaches, that's not a lot to lose really, if you're getting a partial addition of 377 vehicles, and have got the 707s in and all working.
... also , are you sure about the 466s? I cant imagine SE letting the 2cars go.
This is where the picture complicates itself a bit and a bit hazy - I don't know how true the next part is supposed to be. The 376s go to Southern Metro, SDO-equipped and 10 car-capable. 377/3 and 377/4 formations that make 10 car, can be reduced and that frees up further 377s
on top of the ones that are coming over as the 465/9s and 466s go. With all of those 377s, you've removed 375s from the Victoria to Ashford, Charing Cross to Maidstone, Victoria to Gillingham and potentially some other trains - Cannon Street to Ramsgate or Charing Cross to Tunbridge Wells might be an example.
So 376s go, 377s come, 707s stay, and eventually 465s go, and new trains are delivered. The 570 vehicles, at the top end of the tender, will be the correct figure, albeit potentially 568.