Medical courses are grossly competitive, and if it's not something you're that committed to doing then it's likely not going to yield great success.
Any kind of chemical or biological science is available to you. Can be as general as biology, or chemistry, but also more specific to things like neuroscience, or environmental chemistry courses. I did maths, biol, and chem to A2. Heading into fourth year chemistry now.
You still have the option of anything with generic requirements - so even things like law. Strong performance in maths keep economics wide open, psychology is there because you have a strong science foundation.
You've only just started, so i'm sure you'll eliminate a few options as the year goes on - I didn't want to do a maths degree, for instance. I like using maths, but not as a sole degree subject. Turned out I didn't like biology at college, so chemistry ended up being the prime candidate for further study, and obviously I quite liked it.