This comes around every now and then in our airline, usually when the AWOPS part of the sim ends up at LGW. I have canvassed the opinions of other line pilots, trainers and standards guys: the main consensus seems to be that you could *interpret* the rules (as there is no statement as such) to mean you have to taxi forward until you get to an area of 15m lighting but in reality it would be a pointless thing to do, as you've already assessed the visibility as being adequate from where you're starting from, otherwise you wouldn't be taking off. Most of the TREs regard it as an interesting discussion point, rather than a pass/fail item. Practicality is the word here.
Interestingly, I've just looked at the NATS documentation for LGW and it seems like the whole of the runway is now 15m spacing - any LGW regulars able to confirm/refute that? ATC certainly expect you to be able to use M/M3 in LVPs...