You can study postgrad medicine, but it is very competitive- more so than undergrad medicine. If it's an option for you, I would change to do 3 A-levels- I believe for biomedical science this is required, and for medicine it definitely is.
What are your reasons for wanting to do biomed first? In the first few years of a medicine course, you learn a lot of the same content as biomed- pretty sure you are even in the same lectures. If you did both, you would be at uni for 8-9 years (and I also believe you don't get government funding for a second degree..I may be wrong though).