They probably could have been built if desired, the turbostar diesel stock got a few units with mini-buffets, as did 442 and 444, some 319 units?, so I'm pretty sure if they had been requested it could probably have been done.
The impression I get is that the buffet operation in the south central and south east seems to have failed before this generation of stock arrived.
i.e BR and first private operators had already given up operating the buffets on the slam door stock, so they skipped it in the new units.
I offer this related thread with some timelines of when various parts gave up:
Does anyone know why the South Eastern Division did away with buffet cars long before the Central Division did so? I believe that the BEPs disappeared from the South Eastern in about 1980/81 when they were refurbished and converted to CEPs by replacing the buffet car with a former loco-hauled...
www.railforums.co.uk
I too would be very interested to read old bid documents, but I suppose they are all archived
somewhere and will never see the light of day.
The lack of a buffet car is disappointing really, I have always thought better of trains which had one.
I think they genuinely tried under BR to keep things going, but connex probably wasn't much interested.
Though I think, I'd like to see an electrostar with a buffet car or even a dining car.
I'm just trying to think of a scenario where that might happen?
Maybe if the franchise had required a Brighton Belle pullman service?