Come on crew. We have been going through this same dilemma for years - long before Oneworld.
QR do not like award upgrades. They are at the bottom rung on the ladder. If your interest is in a popular high cap route - forget it. If you are attempting upgrades on route that normally struggles for capacity, you have a chance.

If you are chasing a full award booking, QR are much happier, you have much more chance of snagging a seat - as long as you are booking early (like you are) and only looking for 2 seats. The other thing on the LHR route. would be to chase seats on the A330 and the A346. Can't see them coming up on the A380.

Good luck.