I've been debating myself on the future of my favourite Class in the Southern fleet.
They are starting to get PRM modified, bit by bit. Has anyone seen that 455 817 now has LED lights in the interior? They plan to do this to the full fleet. As with the rest of the modifications that need to be done, only minor work is needed to make them comply with PRM requirements, all they need is new door alarms and a call for aid button. The 313's are being done so once these are all complete I'd presume the 455's would follow, these being retained until 2021
If only half the sets are being used from 2018, they could rotate them in and out of use or use the other sets to create an all 8 car Metro service. If Southern wanted to unify their Metro fleet, they could lease the remaining 27 455/8's (ex SWT) and the 456's till the end of the franchise to create an 8/10 car railway on the Metro routes. This could cascade some 377's to Southeastern who are in need of extra stock. Think they'll send the 707's though to Southeastern, unless they want to replace the Networkers in the next franchise.
I hope they don't however end up like the 508's in 2018 and the remaining sets go into store until 2021