As others have already said, Law is seen as a poor A-Level in general, but if you're going to switch either Biology or History then it all depends on what course you wish to do at university.
@Upper Echelons: For all your talk on how History and Biology are for idiots, you seem to ignore the fact that (for those studying science subjects) many students find A-Level Maths incredibly easy. More so than many humanities, at any rate. Of course, you don't care and assume any subject that captures your interest must be the most intellectually rigorous of all subjects, much like a troll.