I've never seen an entry profile for psychology that requires psychology at a-level (apparently it's really different at degree level which is why it's not a requirement at a-level, but don't take my word on it!). If they ask for anything, they'll ask for a science or social science, so you're fine for that.
I definitely think you need to do more research on psychology courses. I don't know about degree level, but at a-level you learn a lot of experiments and evaluation of research, which I don't particularly enjoy.
You should research whether you can use a UK law degree in Australia, as this might sway the decision for you.