When i was in london i used to get the train and the taxi, i hated the bus because a lot of people where in and i couldnt breathe.
Train was very quick so was the taxi, although for taxi you were paying much more but you are alone with you nice seat's comfort!!!
great topic my idea is bus is cheap it depends on are you lover of travel or just want to relax your self if you are hard lover then you must go on bus and by train both are good for traveler it is my idea because I have been in this both way and found wonderful time meet lot of nice people are we talk lot and find more about England nice places and tips from the local people.