I would imagine Thameslink will get something based on the 377s, but hopefully Bombardier will be able to make them work and build them on time.
I'd build them with 2+2 seating throughout too, as with 8 and 12-car train formations and a very regular service it shouldn't need high density seating which is not favourable when the trains aren't full, and restrict room for those standing when they are.
But, what do I know?!
The 319s are supposedly being given a MAJOR overhaul, with disabled toilets, air conditioning and so on - which will, for all intents and purposes, make them seem like brand new trains (maybe they'll also modify the doors to make a more secure seal). If they did the same to the 317s and 321s, they could probably build a pretty decent fleet of EMUs as I believe they're all interchangeable. Bar the cab-ends, and the 317s allowing you to walk from one set to another, they could all be made to look the exact same inside couldn't they?