This is a very serious question and requires a serious answer.First, a word to American members, before you give well meant advice based on the American educational system. The Englsish system is very different!
First, Becky, I imagine that you have just graduated college with a number of A levels. What subjects did you pass in? Were they all music related? Were any of them in biology? If not, I think that you would find it hard to gain admission to uni to read zoology; you would be two years behind.
What instrument(s) do you play? Are you really good at one of them (i.e., have you won any competitions or scholarships)? If not, a career as a performance musician might still elude you even if you take your degree. Or do you wish to study musicology, which would most probably mean teaching music at a school. Does that interest you?
Does your interest in zoology extend to extracurricular study? How many zoology related texts have you purchased on your own? The answer to that question may provide as good an answer as any on how well you would succeed in taking a zoology degree.
I am particularly touched by this question. As a doctoral candidate in English, working on my dissertation, I shared a library carrel with a young woman, and it turned out that we both had taken a BS in biology before switching to the Arts side. One day she remarked how much she had enjoyed zoology and had once thought of becoming a field entomologist but that being admitted to candidacy had "closed that door for ever." This made us both so depressed that we jumped into her car, left "dry" Evanston for Chicago (they share city limits) and got ourselves pissed to the gills!
You, however are eighteen and will complete your degree in three years. You might consider taking your music degree and sitting in on at least a few zoology lectures to see how well you understand/enjoy them. How are your math skills; do you like statistics?
Have fun and let us know what you decide!