Indeed, out of say 12 foreign trips this year I am doing only 2 from NCL, despite being only 7 or 8 miles from the airport. I will use LPL, MAN, DSA, LBA and LGW this year and on at least 2/3 occasions the same route is available from NCL.
You would expect London airports to be cheaper due to competition and number of users however I can't understand how LS seem to be able to charge what they do from NCL when its fraction of what they are charging flying the same routes from EDI and LBA.