Tickets to London are a total biznatch unless you pay three years in advance or do the group save thing and then four of you pay for just two tickets. That's how we had to do it for the expo last year - but the costs don't stop there. For some reason Earl's Court seems to think that a sammich and drink should cost you your very soul in payment...
Don't forget ID - even if you don't look under 18, it's not worth being turned away from the restricted areas just because some security guard decides to check your age and you've got nothing to show for it. Last year they were super strict about it.