Hong Kong to London is the route that Qantas wanted for so many years - it was denied to them because it was a cabotage route - available only to UK or HK based airlines. It was attractive to Qantas because they would have liked to give Australian passengers travelling to London a stopover in HKG, and they would also like to access the important market between HKG and LHR -all those Business passengers. A competitive market, with CX offering four or five flights per day, BA up to three flights per day and also Virgin Atlantic, which had a go at doing twice daily for a short period. AirNZ started at almost the same time as Qantas, and contrary to your comments, the route is not such a strong performer for them either.
If you want to carry local traffic between the city of Hong Kong and London, perhaps a departure at 0735 does not help - it is a long journey from the city to the airport at that time of day with a very early check in time. CX has two day light flights to London, but they leave later in the morning. BA has no daylight flights to London. But, early morning arrival slots at LHR are hard to find.