With very little information to go on I'm assuming it's for 1 pax in Business class?
I see availability on BA to LHR via SIN in J on the 23rd
Code:
Flight details of BA0016
Flight: BA0016
Operated by: British Airways
Departing from: Sydney Kingsford Smith Sat 23 December 2017, 16:30
Arriving at: Heathrow (London) Sun 24 December 2017, 04:55
Stops: 1 (Singapore)
Flying duration: 23hrs 25mins
Aircraft type: Boeing 777 jet
* All times displayed are local times.
Most likely you are having difficulties finding award seat to Europe in December due to the fact that it is peak season and only 7 months out. Award seats don't usually come easy at the best of times and long layovers can be expected. Good hotels have are cheap in Asia so I try to book with a long layover en route to make the journey more pleasurable and relaxing.
There is also availability on MH to LHR in late Dec.
Typically you will find more F availability on CX out of HKG to European Hubs.
Also make sure you search sector by sector on BA award site for better results.