The direct flight is 150k peak, whereas if it's booked as two flights (straight-through or stopover) it will be 60k+105k. Offpeak 125k or 87.5k+50k.

If you are trying to do this with a stopover close to T-355, bear in mind you probably can't add SYD-SIN on it's release, and then add SIN-LON a few days later, i.e. I don't think they'll let you have a booking LON-SYD-SIN even temporarily, and if you did it might not be changeable again to LON-SYD-SIN-LON. So you either chance it by waiting until your SIN-LON date opens up (and for reference a few SYD-SINs are available now, post-release, for Nov19), or you book a straight-through SYD-SIN-LON on release and then hope to move the date of the SIN-LON leg a few days later (for £35).

One more thing! I'm not sure BA actually release 2J SYD-SIN every day (I presume they would say that 2J SYD-LON on BA16 meets their obligation). But as above they do seem to be releasing at least some in Nov19.