A BA routing would produce 40+240 ow unless you find a way to break the journey onto a different flight number for the SIN-SYD leg.
The BA website usually offers a change of flight number in SIN on a LHR-SYD routing, switching between an A380 and a B777, and giving 300TP rather than 240TP for LHR-SYD. Going SIN-PER-SYD gives another 140TP for very little more cash (or did when I investigated a few months ago) and an evening rather than early morning arrival in SYD.