Have not flown JAL so cannot opine on their flights but both BA & JAL use Terminal 2 at NRT and BA F passengers can use the JAL F lounge, so no difference there. On the plus side BA use one of the best gates (in the main terminal, not the satellite and reasonably close to the immigration area and the lounges) but on the minus side whereas JAL have the most convenient check-in desks that are always open BA use the furthest, most remote check-in desks and are extremely punctilious about when they open for business. However many people there are waiting they just stand around in a herd chatting until the precise second that they are due to open check-in.