Whichever way one looks at it (and there's several permutations), the numbers don't add up without a quite substantial long-term slashing of peak services.
Even including 379s, which seems to be pretty much a certainty now, the only way a full 455 & 313 replacement can be made to work and maintaining anything like 2019 service levels is if something else is included in the mix, be that one or more of 377/5, 376 or return of 365 to GN.
379 to GN does allow the GN 387 fleet to be split though, a 50/50 split of the fleet could just about manage 313 replacement and maintaining the bulk of the GN peak extras, but not touching the 455s. This is what my money's on.