The Southern Electric Group suggests that these might be replacing Southern's 455s - apparently GTR have said (according to the Group magazine 'Live Rail') that there will be (a) more 10 car suburban workings and (b) less use of older 455 units circa 2018.
More 10 car will probably be done with the surplus 3car 377/3 and 4 car 377s that are no longer needed on the BML when some services swap to 700s.
I.e. 3 car + 3 car + 4 car like a number of the current 10 car services are.
Far cheaper than new stock.
The proposed reduction in 455 is only about 1/3 of the southern fleet which happens to be roughly the number that could be replaced by surplus 377/3.
