I've actually taken the GAMSAT but luckily I didn't make the right grades for interview. I think my overall mark was over the cutoff (thanks to my brilliant mark on the science paper) but I was short of 5 marks for the essay paper. I'm glad now though!
Anyway! Are you sure you want to do medicine? It's not that great at the moment, have you spoken to some current medics?
The best thing would be is to try and do an Alevel in Chemistry, this is normally the compulsory science A-level when you apply through the normal route. You said you have done some shadowing, thats great, do more! I know it's hard to find people who are willing to let you. Try and do some hospital voluntary work a couple of times a week as well, if you get chatting to some medics, they will probably allow you to shadow them and will talk about their experiences.
Good luck!