I think it is possibly due to either ATC clearance issues, as those flights always use China airspace, and things are really busy, as most Asia to Europe flights take off at those late hours. Heathrow also has a curfew/slot issue, and if the flight time is shorter than expected, they might just wait in HKG and take a slight delay, instead of wasting gas circling around LHR or slowing down midway!

Carfield