I should know this but lacking caffeine, plus a search doesn't seem to give me the answer.

A friend was supposed to be on last night's VLC-LHR which was cancelled. Rebooked by BA onto VLC-LGW with easyJet. Their car was at LHR. Obviously, the taxi between LGW and LHR is claimable, but as far as UK/EU261 delay timing is it the city ("we got you to Gatwick, so that's London and the clock stops") or is it booked destination, i.e. LHR?