Thank you very much to everyone for your insightful comments. I would only have to buy one extra flight (which I can expense most of anyway) so am tempted to try it and see if it makes a difference. At the moment I'm still a pretty new/ moderately frequent flyer, and still in my 20s, so not very flush with extra income (though a lot better off than a couple of years ago when I was still a student). I used to be perfectly happy in economy--BA certainly netted me with the first of those unexpected upgrades!