Many airlines use 737s and 757s with 3-3 seating for flights much longer than most train journeys in the UK.
With incredibly narrow seats that the increasingly broad gauge British public have difficulty fitting into. The last flight I did was Hamburg to Gatwick in the centre seat of an Airbus A320 and it was purgatory, even for a relatively short flight and despite the Airbus having marginally wider seats than a Boeing. My 50" chest frame was wider than the seat, which is why the move towards 2+2 seating in local stock outside the south east is so welcome. Even without the obesity epidemic people today are bigger than in the 1950s and narrow seats just don't cut the mustard.
I must admit the idea of 2+1 seating upstairs on the Aeroliner would appeal, the single seats being ideal for solo travellers used to their own space in their cars. The only drawback I can see is the lack of overhead racks due to the restricted headroom.