As I said, try an expedia booking (oneway, SIN-LHR, BA) on your date and when you see the BA12 flight there's a "Seat preview" link which will show you which seats are free/taken. There are plenty free so far.

All the seats are lie-flat, and I don't think there's much danger of you not being able to get seats together when check-in opens (online 24hrs before the flight). However I would try explaining the situation to QF and they may pay/beg BA to assign you seats. I doubt that would be the upper-deck though - it's a nice place to be but I can't see you being able to guarantee being up there unless you pay for it.