Just book it. It's a good deal, if I needed to get to London I'd take that deal..
Expedia shows the fare buckets as J for BNE-SYD and a combination of A/F for SYD-SIN-LHR for $7.5k, which is First for the long haul. I suspect a fare error as its about $4-5k more on the BA site.
I think the...