I have also done this route many times (in WT+,J and F), and with many different combinations of flights. My favorite is the BA LHR-HKG flight (BA27 2215) connecting onto QF128 HKG-SYD.

My reasons are that via HKG, you get to enjoy the hospitality of T5 (assuming you have lounge/CCR access rights), plus you get to enjoy the CX wing/pier lounges in HKG. From a flight perspective the 12hr and 9.5hr flight durations are more even (c/f BKK or SIN) which tends to suit my sleeping patterns better. I find the 6-7 hr hop from SIN-SYD too short to get some good sleep once meals etc have been had.