Underage concerts in London?

When I was your age I didn't have any problem going to gigs. I think it's the size of the gig that's the problem. I used to go to a lot of rock and indie gigs at venues like Brixton Academy, The Forum, Camden Underworld, Electric Ballroom, Mean Fiddler/Astoria, Koko and 90% of the time the gigs were only 14+. I suggest checking out the listings of those venues and choosing something you like out of them. If you haven't heard of the bands that are playing then myspace or youtube them. It's a great way to discover new music too. Good luck.