Thanks, CWS!
Cash part of the tickets was £1.80 each and 90p for the kid.
Now that I understand our position better I'll feel more prepared approaching JAL for clarification. Our travel is off season and each of their B777s has 500 seats, will be interesting to see how supportive they are of the economies on the smaller Ryukyu Islands (the plan B will be returning to Naha a day earlier)