Ive just finished studying Business, Psychology and Sociology. I am now going on to study business psychology at university. The fact that youve done ok, you dont neccessarily need to start from scratch on all you're AS subjects, maybe swap some and carry on some for A2? but tbh unis arent interested on the subjects you study, more the grades and you can express your interest in your personal statement.
I enjoyed psychology (hence why I am carrying it on) background reading is a help and learning exam technique is important. any questions feel free to ask. As for other subjects that will help, I would say there are links in sociology, business, maths and biology. Again specific subjects arent needed (you dont even need an A level psychology to do it as a degree)