The issue with buses at T5 is primarily due to the BAA not building the terminal on time or to the required specification. T5C is still not ready and T5B was cut back to save money when it was sold to the spanish. That is why the end jetty of T5A is the longest in the world.
BA have, as a result, to coach over 20% of their operation at their home base and in the newest terminal in Europe!!
Add to this the mess that is LHR generally. 94% of all aircraft movements take place between the two runways and these are planned to be used at almost their maximum allowed rate for most of the day.
Then add ludicrous 1950/60 noise regulations limiting use of runways and preventing sensible dual use all day long.
Throw in a bit of bad weather....it does not matter what type, snow rain wind, sunshine and the landing rate will plunge from 48/hr to often less than 40. Indeed on most days of the year some form of regulation is applied to the landing rate, which in turn impacts on turn rounds and therefore on departures and therfore on stand use and planning.
For good measure add BAA incompetance. They train to create queues at this they are the best in the world. But security screening is woeful.
Upshot is BA get clobbered time and time again.
I like BA, fly BA and will knock them when required, but bottom line here is they operate at the worst major airport in Europe if not gloablly and which should be bulldozed and relocated as soon as possible and then run by and airport company and not a retail consortium.