Better standardisation of fleet purchases.
There should be a 'standard' DMU / EMU body design for the whole of the rail network. Manufacturers could compete to produce trainsets based on such standard bodies. It is hopelessly uneconomic to procure as is done now, with trains desigend for one or two routes. Limited efforts have been made before such as AC / DC designs for southern services, but what is needed is standard designs, where bogies are essentially the same, but differing capacities of traction motor can be fitted. All DEMU / EMU's should have through corridors. Yes, it may INCREASE the initial cost a little, but long term it REDUCES the cost - eg TPE 185 units

(great units except seats are too hard

) often connected into pairs, no corridor connection two guards or revenue loss due to non-fare-paying. Because the control systems differ between classes, they cannot be swapped around to meet changing demand. To cope with increased capacity, extra coaches are often required, but once the production line is finished its hugely expensive to restart - this would be less of a problem if there were standard basic designs. It is not particularly obvious why 8 coach train sets in south which need to be made 12 should be dumped onto the rest of the country and south gets new 12 coach sets - even if the speed needs upping to 110 or 125, it should be possible to simply switch supply equipment and brakes, not need a new class. It should also be made irrelevant whether its a DMU or EMU being ordered - it should be a simple matter to convert from one to the other as circumstances dictate- ie a week or two, at most, not forever.
Yes, the loading gauge may be better in some parts of the country, which might allow greater room onboard with disrete designs, but its simply not worth having lots of separate classes with little in the way of common parts and training requirements. It is the wrong approach. Years ago, it was impractical technically to produce a single emu/dmu design, but I don't think it should be now, it just needs someone to sit down and specify an adaptable body shell framework. Door opening can be altered dependent on use ie heavy commuter or long distance. The interiors would differ of course to provide more luxury for long distance services, but the shell would be the same.
IEP is also a complete waste of money. Refurbish 225's - the biggest customer experience improvement would be to replace those crap rock-hard seats. Introduce 140MPH signalling on ECML, maybe introduce tilt for Scotish services use money saved on IEP to electrify to aberdeen, inverness, hull, harrogate. No need for IEP then.