I think it depends on whether you're interested in it or you feel like you'll get good marks in it. in terms of medicine i'm not too sure whether it's necessary or not. If you choose it and you don't like it then you can always drop.
Actually i know someone who wanted to do med and had a great interest in biology but he chose other subjects over it because he performed way better in those ones...
I'm assuming that you are in australia and want to study abroad. If so hsc bio is ok. It dosen't scale as well as I would like but meh.
That said if you are in brittan now I have no Idea about bio over there :?