SKA, I really do sympathise with your predicament. Over the years, I've spent quite a few hours discussing with students similar situations as you've outlined.
If you really don't like biomedical sciences, I think you need to ask yourself exactly what it is that you dislike about it. Would you prefer to study the life sciences or biomedical sciences? As you don't seem to dislike the laboratory environment per se, it may well be that moving to the biological sciences may suit you better.
You've not given us much information on what biological sciences are available at your academic institution. This could be important as you may be able to study, say, botany, zoology, biochemistry or ecology as dedicated subjects. Does one of these appeal to you more than another?
In my own university, I nearly always suggest to a student that he or she has an informal chat with their tutor to discuss the options open them. Tutors often have they're "finger on the pulse" when it comes to things like this and will give you sound advice. You can speak to them in confidence and they're often better placed to arrange transfers between courses as well - in my own department I always insist that I have young, postgraduate PhD or D.Phil tutors to hand for this very purpose. After all, not everyone feels they can cry on the shoulder of a fuddy-duddy professor and dean like me!