Besides the Southampton stopper [...] is often a 450. I'd much rather get a GWR 158, or even an Electrostar!
Good lord, really? What don't you like about 450s?
Unlike the 158s they offer acceptable legroom in all seats (I'm only 6'1" but find I cannot move my legs or use the seatback table in a 158). And of course have none of the rattle and roar and smell of the diesel...
I'd also choose a 158 over an Electrostar... The latter has harder seats, and too many table seats where once again you can't move your legs without kicking the person opposite you. I find the interiors gloomy and claustrophobic too.
450s are light, airy and welcoming, with a good balance of airline and facing seating. The only place where a 450 loses out is its noisy air conditioning, in my view... Please First Group, don't 'refurbish' the interiors into some gloomy colour scheme that changes that...