I notice that the main coach operator between LTN and central London is Greenline / easybus. I notice also that the turn-up-and-go fares are very expensive, given the length of journey, and that those booked in advance on the easybus website are only a bit cheaper unless you are prepared to travel very off peak. Yes, there is also a train service, but when a coach service is more expensive than a train, it usually indicates something has gone wrong with supply/demand. In particular, the 757 Greenline coach looks to be rather monopolistic in its pricing.
Normally, when coach fares are both high and the coaches are full, another operator becomes tempted to have a piece of the action, and starts up a competitor service.
Is there anything that would stop National Express or Stagecoach doing something to rival Greenline ? I'm just puzzled as to why the existing coach monopoly carries on unchanged.