Dr. HFH where did you get those numbers from? The great circle distance SYD-LHR is 10573 and HKG-LHR is 5994 miles. The numbers will be larger if measured in km and smaller if measured in nautical miles.
Flight time is decided by many factors. Some are naturally occurring such weather, the rotation of the earth, and in the Northern hemisphere the Jet-stream, even temperature. Other causes are man made such as fight restrictions, wars and unrest etc. That is just a few reasons rhere are many many more. Like you I would like to know how these and many other factors affect flight time.