You can change your choices even one month after you start the actual course. Just call/send an email to the college and tell them of the changes. It's not too late whatsoever.
It's a shame you're dropping politics, I love the subject. As for law, a lot of people tell me that universities don't actually like students studying law at A-Levels. I don't know if this purely a myth, but I've been told this so many times, so it must stand some ground. Anyone correct me if I'm wrong. I don't do psychology, so I can't tell you, but I know it has some mathematical aspect to it (hypothesis testing I believe).