AA is my choice. In two years of living in London, I did numerous 6-7 RTs on AA, 1 on DL and 1 on BA (all Y), and one on VS in J. Ignoring the VS experience, AA was an easy choice. Same frequency as BA JFK-LHR, acceptable food, comfortable seats (especially if one gets 31A/B/H/J as an elite on the 777), Fullers London Pride [beer] on the westbounds. The flight is short enough that after eating and a bit of reading, there is just enough time for a nap.