I've encountered this sort of thing a few times mostly on the buses going up Sydenham Hill from opposite the Horniman Triangle and I am my children have been left in the freezing cold and pouring rain. Usually the bus drivers do slow down and stop and then tell me I can't get on but also sometimes they assume that your buggy can't fit on and have been pleasantly surprised when it does.
What happened to you was unacceptable but it might be part of whats been going on recently in respect of consultations about buggies being refused in case they take up a wheelchair space. Probably worth enquiring about that. To be honest the biggest problem I have had with buses has been some of the other passengers who refused to move from the buggy area even though there were seats available elsewhere.

There is no easy solution to all of this; many people object to buggies getting onto buses and don't go out of their way to accommodate them. At peak hours its just not worth the trouble. I have been on the single door p4 buses with a buggy with people hurling abuse when I had to get them all off the bus so that we could disembark at our stop. The best examples of good practice seem to be on the Brighton buses which seem to be designed to take 4 or more buggies at a time and the drivers have a much more friendly attitude.

After all this is not about denying access to a buggy, its denying access to young children, who do have the same rights as everyone else to public transport but suffer considerable discrimination.