They already are doing this.
I think that is one element, but people also want to make these journeys quickly. If a journey time is too long, even a free bus becomes very unattractive. The recently ceased Chester to London proved that. 7 hour journey because of the nonsense routing. Same is happening now with the Liverpool to London and Liverpool to Birmingham corridors, forcing people to go via Manchester was one of the biggest reasons why Megabus kept failing in Liverpool. Now NatEx is doing the same.
What previous premium options were there in the past? Megabus Gold and Sleeper I can remember. Gold meant some people sitting backwards which I don't think is very popular on coaches, and the sleeper was an experience rather than normal coach (with lots more expense too because of the conversion from day>night and any costs associated with the linen and stuff).
I think something would work if you had something similar to Regiojets coaches which run London to Prague. 1+2 seating upstairs, decent leg room and recline for the longer journeys, onboard coffee machine for passengers to use and vending machine etc. High spec with lots of luxuries, but still largely a normal coach.
