Well, you won't HAVE to see her, but it's going to be pretty difficult to suddenly start ignoring her - and you're right, if you have a best friend already then there's no real impetus to go out and make new ones. I deliberately didn't go to the same uni as one of my best friends because I wanted a completely new start, and I haven't regretted that decision yet.
Are you doing the same subjects or living in the same halls (I s'pose it's too early to know about accommodation)? Because if not, you'll have different friends in both areas and you just won't see much of each other because you'll be caught up in your own course and halls, meeting new people in those situations. Friends who I know have deliberately gone to uni together have almost always ended up drifting apart simply because they didn't have as much time (or, it has to be said, inclination) to spend with each other as they had before.
However, if you're that worried about being stuck with her, could you consider going to a different uni?