I know quite a few people who have wanted to go down that route and as what bethie said, it took them years to get accepted into the schools. My advice from which would be the people i know's perspective is to get as much (life) experience as possible. Audition for pretty much as many drama companies now as possible, a great company to audition for is NYT (National Youth Theatre). It's a great way to get noticed and i know drama schools definitely consider it, they all hype it up at our college.
http://www.nyt.org.uk/ But if not, then do volunteer work for a year, anything that would boost your cv, you need to be very prepared for whatever gets thrown at you. Best of luck to you though.