I believe part of the difference in fare is that JAL YQ is, believe it or not, higher than BA

I have only ever done one sector in JAL F and it was memorable for the food alone. The service was impeccable and I would hazard a guess that service standards would be more consistent on JAL than BA which can vary greatly.

I think on the LHR route JAL use the older JAL F seat, not the new Square suite.

The sector I flew was NRT-SIN (when they still offered F on this route) and was four years ago. I ate the best steak I have ever eaten in the air on that flight.

If you care to you can read about the flight here