I did MK to LHR quite a few times in the past and never took the train. National Express runs several direct services from MK coach station, relatively cheap tickets if booked in advance, journey takes roughly 2 hours, there are only a couple of stops (LTN and Hemel Hempstead or LTN only for certain services) before reaching LHR T2/3/5. Of course one should take the traffic into consideration when considering this option, M1/M25 are notoriously bad. As for the other option (i.e. Watford) which had been previously suggested, I think it is worth mentioning that this is a local bus service (starting in Hertford/Welwyn Garden City/Hatfield or somewhere else in Hertfordshire) and can get crowded at times.

G