Well I will base my decisions on the following factors --
1. Does earning miles matter to you? If you are getting miles on the BA
program, it will be nice to keep that going; but if you don't care, JAL mileage bank is not too bad, especially after the last round of devaluation by BA EC
2. I think JAL service is comparable, if not better than British Airways. On the NRT
flight, you get the brand new solo suites up front, which is much nicer than BA
'dated' suites. Food wise... JL
has a nice Japanese selection but a nice western menu as well.
3. JAL considered LHR
as one of its top routes, so service will be the top notch. Give it a try!