Can you study other things while pursuing a course at a UK university?

I understand that at UK universities you generally one thing for three years. It might not be the same at every university, but using Oxbridge as an example (since it seems fairly rigid in it's systems) if you were to apply and get accepted and then start a course of study at either Oxford or Cambridge, say in law, or in history, could you also take elective courses in other subjects, like in math or a physics or anthropology if you were prepared for that course? In the United States you declare majors, but can take any other courses you want on the side as long as you take enough courses to satisfy your major.