If you're definitely sure you're going for a music degree, I would guess at least one other essay subject would be helpful, especially English Literature if that's what interests you. And for a fourth - anything you like really. A language would have a tenuous connection to music, or a science for a contrast, but really I don't think most unis mind. I would look at some courses to see their specific opinions are though. What's more important is to do a lot of extra work on theory and performance outside of school - Grade 8 theory or above would be really helpful before starting a uni course. Plus as much extra listening experience to widen your knowledge as possible - music is one of those careers where you can really decide what you want to learn yourself.
I confess jobs wise I can't be much help - all the professionals working in music I know would be teachers of one kind or another.