I find that BA crew on long haul flights are almost always at least OK, and occasionally very good. I can't discern any particular patterns, except that I do feel that a small CW cabin on, say, the 787 (and I appreciate that the question is not confined to Preimum classes) will sometimes feel to me like it has more attentive service that a 380. Also on a night flight I tend to sleep and therefore interact much less with the crew, making it harder for them to impress (in either a positive or negative sense). Also, the difference between an ok crew and a great crew can often be one or two little things, like a crew member that is just a little bit more attentive, or smiles or makes a funny comment, which makes the experience more personal or memorable. I don't recall a truly awful BA crew, though there have been a handful who were clearly going through the motions- but that's ok too imo.