There's not really much of an issue with non-corridor-connected stock working in multiple. Almost every local TOC around London has such stock- GA also have the 360s, c2cs fleet is entirely 357s, FCC have 365s, 321s and 319s, LM have 321s, Chiltern's 165, 168 and 172 fleets, FGWs 165, 166 and 180s, Southern's 455s and 456s, SE's 465, 466 and 376 fleets. Heathrow Connect and Express use non-corridor connected units. LO only use single units, and at SWT, the connection on 455s and 458s is for crew use only.
To replace the 321s with something with corridor connections, you'd need 94 new units- 376 carriages. If the connection was the reason you were doing it, you'd need a further 21 units/84 carriages to replace the 360s.
(noticably, if you swapped the 360s to FCC and LM for their 321s, you'd only be one unit short at GA- which would probably OK on fleet-use. Could probably get the ones that went to LM converted to have 350 cabs. You'd need a damn good refurb of the 321s to keep the likes of Ginger from exploding though)