Thanks for posting this before I included KEF in a xONEx ^

It is up to the airline how they calculate, although it should be reasonable. Note GCM is not 100% accurate but normally within a few miles.

While 1182 is considerably shorter than 1205 LGW-KEF (enough that it is reasonable to ask QF how they are calculating it as opposed to 1200 which I wouldn't query). However, I note from GCM that using city codes instead of airports LON-REK is 1175, even shorter still. Therefore while not happy with being just under zone 3 limit it doesn't look ridiculous.

It would be interesting to see how it is calculated and whether the method is consistent with other routes also open to similar variations (such as between LON and NYC or LA and NYC).