And NB that is the distance to the capital city of the country you are flying to, beyond any connections less than 24hrs along the way.

e.g. LHR-xAMS-xSIN-SYD = Band D = £110 in J.

This can be used to your advantage:

e.g. if flying to Hawaii it counts as Band B (because LHR-IAD = 3600miles = Band B), despite your much longer flight!