The drivers let people that haven't booked on for cash while people who have booked are refused as there is no space.
My 83 year old mother has just tried to get on the bus she booked back from Gatwick with a friend at the right time and the bus driver said there was no room and he would have to drop people off first and come back. This was after he lied about being the correct bus in the first place and he never came back.
When I put her on it in Earls Court the driver crammed and extra people who hadn't booked and took cash off them. The whole thing is a shambles and you literally have to fight to get on the bus you booked.