I don't think law firms favour those doing other courses, they just don't favour those doing a law course. So there isn't a specific "benefit" to doing a law degree over something else in terms of getting a training contract/pupillage offer. However, it does make things slightly more straightforward as you will probably be in a better position to take the SQE without shelling out for a prep course, and for the Bar route you wouldn't need to do a GDL. So if you're certain you want to go into law, it's fine. If you aren't 100% certain of law as a career, I'd probably suggest doing another subject just as then you still have law (and anything else) as an option but you won't be locked into a (by all accounts) very dry degree which has a low first rate, for no reason.