Hi,
I study chemistry, physics, biology and maths at AS-Level and I want to study medicine too. I think that your chosen ASs for the moment are good and provide a lot of variety and would allow you to get into medicine at uni. However, I have to say that although my subjects can be quite hectic at times, I do find that there are so many links between all of the subjects which really does help, so I would encourage you to think about physics, which you did mention earlier.
At the end of the day though, I think that you should pick the subjects you enjoy most and know you will get good grades in because after all, you could end up spending 2 years studying a subject you hate.