It certainly happens, and must be annoying when it does. But BA now has more gates than was the case at T1/2/4 in the old days.
BAA is responsible for adequate provision of gates and has failed to provide adequate capacity.
I have been lucky with bussing, only having two flights over the past year which have required one.
I do think BA should do more to segregate its F pax when bussing is required.
Lufthansa does this perfectly with chauffered transfers to the aircraft. I do not think this is possibly with the volume of BA F pax, but a “nicer” bus should not be too hard to manage for those who turn up in good time, perhaps to a central F embarkation point.
Perhaps some of the money saved from the planned cost reductions elsewhere could be used to refurbish some of the (almost) brand new eco-friendly bus fleet once T5C opens and fewer buses are needed.