Yeah outside of the centre it can get a bit unpleasant but the centre itself is all right. The Eye is good and there is all the other tourist attractions in and around the centre (most are pretty crap). You might want to take a look at this website: http://www.visitlondon.com/
it should give you all you need to know. On the pub front there will be a pub that will sell you if you are underage but most of the ones alongside the Thames won't (my friend was refused last year). If you have a fake ID that is passable and shows you are over 18 (and you look old enough), alcohol shouldn't be a problem.