Admittedly my experience is a good more than a 13 years ago, but I've never known anyone in London who used a fake id to drink. While the age at which one can buy alcohol is 18 (there's no prohibitions against drinking it, just buying it) it's so loosely enforced that it's generally just a matter of going to an off licence down the road if the first one asks for ID, so there's almost no point at all in getting a fake ID. Added to that is that there isn't any form of ID other than passports or driving licences which would have a data of birth on them, and an under 21 yr old not having either wouldn't be particularly unusual anyway.