I've used BA25 around 3 times a year for the past 15+ years. I can probably count the number of on time departures on the fingers of one hand. Ok possibly a slight exaggeration but I can also only recall a few where we have actually arrived in HK on time (although until fairly recently HK ATC sent you half way to the Philippines before turning back to HKG). Typical push back delay is only 30 or so minutes, which then seems to extend due to the departure congestion at LHR around 7pm.
Various reasons are given; the most frequent being the location and removal of baggage of pax who have not boarded the flight. That suggests missing connecting pax are the major reason.
And then the was the important chap sitting behind me who announced that he had left his Blackberry in the lounge and simply couldn't travel without it as it contained his itinerary and the addresses of those he was due to meet. Eventually a compromise was reached where a lounge staff member would remove the SIM from said device and bring it to him (bringing the device itself on board was a security no-no). Another 30 minute delay (it was a long walk from the then T2 lounge)...
In March the inbound 380 had an encounter with a bird on final which led to a delayed departure on a 777 at 12.00 the following day. However in my experience technical delays have been minimal.
All anecdotal and undocumented of course. Correction/clarification from those who are better informed most welcome.
But despite these delays this earlier flight is still preferable to the later BA27 as you get several hours of daylight exposure on arrival which seems to help the body clock adjust a tad quicker.