I know some of this is fantasy but complete and utter standardisation will never happen, especially with the rate of technological development today. For example in the Siemens Desiro UK lineup, 350 to 380 to Desiro City, whilst with Bombardier, 357 to 375/377 to 379 to Aventra. This doesn't just happen on British shores; the German suburban DBAG 423 MegaClass (ADTranz/Bombardier/Alstom) was superseded by the 422 in 2007/8, which has already been replaced by the 430.
Furthermore, let's say that no Desiros made it to UK shores and instead, all services that these run today were provided by Electrostars (375/377). A vital part shared by every trainset is found to be at risk of damage, and all fleets are grounded until the part is changed. The network would come to a standstill. However, if this happened today with Desiros in the mix, this wouldn't be so much of a problem.
And however much I regret to say this, the 507s and 508s on Merseyrail, and the 313s on GN Inner Suburban to Moorgate, may not be replaced by a common 378-type, with 2+2 seating obviously.