I'm having trouble finding a date combination that gets that super cheap fare on BNE/HKG/LHR. Cheapest I can find is about $1,900, which of course isn't promo.
If you want to find a good date combo, looks like you've got quite a bit of trial and error (maybe it is easier from other capital cities).
Just a note - my dummy booking on the $1,900 fare would book into N class ("Economy SuperSaver"). I'm not sure what these other super duper cheap fares book into, but N class does not earn any points or status credits on QFF (and indeed doesn't earn anything either on Asia Miles). Again, don't know what these super cheap fares book into, and it's still a great fare even if you forgo the mileage and status credits.
The next fare class up ("Economy Saver") appears to book into V. Not sure if it's a small differential on top of the sale fare; just for reference, my dummy booking as before in V would raise the fare to about $2,200.