I'll be biased as I do like them a lot, but I really do think we should be buying more FLIRT style trains such as Class 755s
Very comfortable, surprisingly quiet, and of course the options for batteries and OLE alongside the diesel powerplant makes them a very attractive prospect. I also think there could be an opportunity for making them DC-compatible for replacing some Southern/Southeastern units, helped by Stadler already having experience with the Merseyrail 777s.
I genuinely do think 231s or 755/756s would be a very worthy replacement for the venerable 159s, and the 100mph top speed plus the better acceleration could make them viable replacements for Turbostars as well imo.
I personally think they're a bit rubbish compared to their European counterparts (of which I'm a huge fan) and that they haven't scaled down well at all* - I find them cramped and clumsily laid out. I would love to see what Siemens and Alstom could offer, I think it could potentially be a lot better. Something like a 3 car 24m Aventra with a low floor dip between the bogies of the centre vehicle could to me be a very good compromise indeed (the diesel version being able to have a larger engine under each end vehicle, say 750hp, but none under the middle one). It's going to be hard to do that meaningfully in a 20m unit, though - I think 20m unit land might be better off doing the work to be able to get longer distance services to 24m units (as is already the case on SWR), while for low speed innersuburbans a unit that is low floor throughout with smaller wheels for a lower top speed like the near-perfect London Underground S-stock would be worth considering.
The other downside is that the FLIRT design puts a lot of stuff above the floor that doesn't need to go there in a hybrid unit with only some vehicles low floor, which wastes platform length. It's like that because it's designed for European style *really* low platforms, but if you just want to get down from 1100mm to 960mm you don't need those compromises, you can still put stuff underneath - you're only making the floor about 15cm lower which is hardly anything.
That might I suppose get you two or even three separate tenders.
* Most notably the Class 777 is in my view absolute junk from start to finish, a hugely expensive load of scrap, and I loved the idea until I set foot on one, but while the Anglian and Welsh units are doubtless an upgrade on rotting old 15x I just think they are not particularly good designs of new unit, with the multiple levels of floor inside with awkward ramps, odd window sizes, "escape windows" and cramped seating (with limited scope to alter the layout) really not doing them any favours. Yes, on Greater Anglia the Class 720 is worse, but to be honest that would be worse than an open flat wagon pulled by a diesel shunter - and the actual unit isn't bad, just the seating - the 730 is pretty OK.