A-levels are the exams given at the end of the British Junior and Senior year of high school in England.
I took my A-levels a decade ago back in Cambridge, England as it happens. However, they're not administered by Cambridge, they're administered by the Midlands Education Authority or some such.
Perhaps you mean the S-levels. Special, additional exams required for college entry to the University of Cambridge and Oxford. Those are administered by Cambridge.
A-levels are easy (on par with with an American pre-req at the university level). However, THEY WILL NOT BE VERIFIED BY AMCAS and thus won't count towards your pre-reqs for American medical school.
S-levels are the hardest exams I've ever taken in my life (besides the International Math Olympiad finals). I took the Math level 2 and 3 S-level though, which was probably masochistic of me. They are more difficult than any american graduate exam, but again, THEY WILL NOT BE VERIFIED BY AMCAS so it doesn't really matter.
If you want to practice medicine in the states, you should just go to an American University. I wish I had.