Flapping I can't answer your question as to whether you disembark at HKG, as I have not done that route but the airline (or agent) can tell you.
As to the late departure from LHR, bear in mind that EVERY carrier now builds time delays into their schedule. On even a short domestic sector, there is time built in. If the block time is typically one hour, then up to 15 minutes will be added in the schedule. This allows them to arrive on time and have good statistics for publicity.
As to getting a three seats to yourself? Impossible to know until the moment the doors close but every carrier works towards having a 100% occupancy. The computer programmes that calculate how many people will buy a ticket but not show up - and consequently how many extra tickets they can sell - is a fine art and works very well. You have to assume that you will not have any seats to play with.