JFK-LHR is a confusing one because they use the 777-200 and 777-300 on this route. All 777-300 are 3 class (F/J/Y) while the 777-200 have either J & Y if they have been converted or F/J/Y if they have not been converted. AA is in the process of converting the 777-200 to J & Y only. In the past when the 777-200 was 3 class. there were usually pretty good odds of getting F as a nonrev but at a cost, especially departing LHR. The configuration was 16 F/37 J/194 Y. Haven't tried to nonrev to LHR on the -300 or converted -200 but with only 8 seats in F on the -300, the odds aren't so great anymore. Aircraft subs happen a lot so if they were planning on using a 3 class 777-200 then change to a 2 class, you're screwed. Sometimes, aircraft subs are beneficial. Once flying from HNL-SFO, I was able to get F because they used a 767-300 instead of the scheduled 757.