I know that the AT300 is a fully modular design. What I'm saying is that I don't expect it to be worthwhile to create another AT300 configuration for use for only a few years. See, there's not a vast market for 125mph capable commuter trains with diesel engines, as TP North is really the only route where they're needed. The only other 125mph diesels lines are the GWML, which is having all its main line commuter services into London wired completely, the MML, which is already wired on the fasts to Bedford and will be wired beyond around the same time as TP North, and XC which doesn't actually need commuter trains capable of 125mph. As there has been no clear improvement in the ability to fit powerful diesel engines to AT300 carriages without needing to raise the floor, having a commuter door layout isn't going to be any easier either. You don't want to have a particularly high step up, and unlike an end-doored design you can't hide the raised floor with a slope at either end.
Yes, it's modular, therefore it can be tailored to a particular need at minimal extra cost. What's your problem about this?
Why do you think the trains would only be used by TPE for a few years? Who says they have to move anywhere else anyway?
Take off the engine rafts and you have electric trains that can stay on the same routes for years - assuming you don't want to keep the diesels for diversionary purposes via unwired routes and never mind that, as I keep saying, there is no sign yet of a plan to wire Scarborough or Middlesbrough, so you will continue to need plenty of the bi-modes to serve them for a long time after the TPE core is wired.
If they go for a 23m bodyshell - and the 185s clock in at 23.763m - that should give them enough extra room to play with, compared with a 20m Class 395 bodyshell, to keep the doors in the 'low-floor' areas at the ends of coaches and accommodate shallow ramps to the raised central section over the engine.
As for the notion that XC doesn't need 125mph commuter trains, do you ever travel on their services at peak times in and out of Birmingham, Manchester, Bristol, Oxford and Reading? A lot of XC's passengers would be delighted by an influx of trains like this.