The only fly in the ointment is the mainline rolling stock is very old (as in it was brought in from GWR after one service life) and we’re miles from any decent hills.
Eh? The mk3 coaches are infinitely nicer than most commuter rolling stock, and they didn’t come from Great Western, who are still using their mk3s in HSTs. The loco hauled mk3s are different, Abellio Greater Anglia have had some for years, others came from Virgin after they introduced Voyagers and then later Pendolinos.
Plenty of them still in service with FGW, East Coast, Cross Country, EastMidlands Trains and Virgin West Coast, plus derivatives in EMUs for most of the major TOCs.