One thing I would say is that you shouldn't go into it think that you know what you're going to drop after AS. You might think it makes sense, but you don't know whether you're going to enjoy all of your courses yet, or what grades you're going to get. If you know the uni you want to go to requires AAA, and you got an A in the subject you want to drop but a C in the one you want to keep, you'd be making things very difficult for yourself..