Would you split across two days? That’s your best bet at LHR-SIN-SYD in CW. Other variations would be LHR-SIN (2 on the A380 and / on the 777) with / in Y to SYD on the 778 and 2 in club. BA guarantee to release 2 Club and 4 Traveller seats on every flight. Nothing is guaranteed in F or WT+. There might be J availability on Qantas from SIN to SYD, but your 2-4-1 will only be useful for BA metal.

it might be best to secure something, plans can be changed later for £35. Or cancelled if a great sale fare comes along.