If there's any money to lengthen platforms, I reckon that Crewe, Warrington BQ, Wigan NW, Preston, Lancaster and Carlisle could all have some 400m platforms without too much disruption.
Warrington BQ
Up: 253
Down: 260
Wigan WN
Up: 274
Down: 256
Preston
Up: 269
Down: 362
Lancaster
Up: 253
Down: 258
Carlisle
Up: 349
Down: 314
A quick look at those stations and you're looking at some significant lengthening works to make them 400m. 300m would be much more reasonable and without a doubt a lot less disruptive and more feasible, on the plus side Carlisle can already accommodate them! Since the infrastructure already supports 250m trains at all those stations it might make sense (if we are going to split and join services up to 400m) to run 250m and 150m sets ?
275m + 125m = 400m, so you could still pair them up for Brimingham services (or Manchester ones if it gets 400m platforms at some point over the next 40 years). going for 300 and 150 meter ones doesn't allow that, so would limit the services to 300m even at the places where the infrastructure supports 400m
The problem still remains that splitting and joining will just end up with a much more unnecessary complication rail network to operate. Personally I don't even think splitting and joining much shorter commuter services is a good idea, it only causes confusion amongst passengers what is going on.
To insist upon running 400m services between London and Birmingham (275 + 125 units) you end up with either less or marginally more capacity upon the existing network above Crewe were presumably under your suggestion these services would also have to split, which I don't believe is a good idea.
Another problem with splitting and joining 275 & 125m is that you couldn't stack two 275m units on the same platform. Now that is a benefit of having units 200m or less. Considering Euston is likely to have significantly less platforms, not being able to stack is just going to cause more problems!
Personally I wouldnt bother stacking but if you start running around 125m units its inveitable you will!
I dread to think of the people who will end up planning HS2 services!
Lets keep it simple and all HS2 trains the same length!