Every time I've operated the HKG-LHR sector I have had to hold the aircraft on the gate or remote hold/slow taxy due to the slot pairings and arrival constraints.
The difficulty of getting departure slots out of HKG tying up perfectly with the transit times over Russia and into LHR with a scheduled arrival time of 0615am local are to primarily to blame. The stand allocation at HKG is premium and,
AFAIK, the only times we can get.
If we depart too early, even flying the route at cost index 0 we would arrive only to either land before the night jet ban at LHR (landing before 0602 local) or have to take up the hold until after 0602 local, which, for fuel purposes, is why we often either wait on the gate (if allowed) or push and hold.
This particular flight is known for the mismatch of departure time, flight time and arrival slot but, despite departing late, I have generally always arrived either ahead of schedule or on schedule due to the block time versus the flight time giving plenty of flexibility to 'catch up'.
Hope that helps.