I sort of know where you're coming from; I wanted to do a degree in Molecular Biology for which you needed biology but only had Human Biology as an A level. My uni was ok with that provided I did well in it. I also know someone who had to take a kind of crash course module in the first year, then convert to the actual degree that she wanted to do.
Different universities have different policies on this kind of thing though, so your best bet is to talk to your tutors and universities that you're interested in and see what they think would be best for you.