One more thing to consider, Puchu, is the likelihood of your "bunch of empty seats in economy" actually coming true. We've all been on flights where there have been loads of empty seats and stretching out has been possible, but that has not been my experience of late. I mostly fly trans-pac so have no idea what the loads are like to the UK, but if you book economy thinking that if (read: when) your upgrade doesn't clear, you can always toddle down the back and push up a bunch of armrests, you may be in for a pretty uncomfortable flight when you find that eamus, mrs eamus, the little miss eamus and hundreds of others like us are jammed into every nook and cranny back there. You may want to send some smoke signals up querying load factors. In the brave new world of everyone flying, in your situation I would (in order) look for award F, award J, pay for J, pay for WT+ on BA, or plan on misery. Upgrades are hard to come by, and getting harder. If J is outside your budget, crack open the search function for the QF/AY, QF/AF, DONE4 or Circle Explorer tickets.