I would not worry about a -level choice. Unlike most degrees, Law is a degree where you do not have to study specific subjects. Your skills of analysis in chem and bio coupled with your essay writing in psycology are good enough! So do not worry about that. I am studying chem, gov and pol, history and spanish at A2 and hope to study law!
Also, shadowing what the others have said above, make sure that you really want to study law. Its competitive and you need to show you passion!!
Good luck!