I would say that if you are aiming for Oxbridge for either Medicine or Law (as your original post states), then Biology, Chemistry, Maths, and History would probably be the most ideal combination.
I would definitely advise NOT doing 2 sciences and 2 arts, since that wouldn't give you the strength of scientific background that other medical applicants would have. Medicine is more picky about what subjects you do than Law. Technically all sciences would be fine for Law, but I think having one essay subject is probably a good idea.