Do maths, further maths, bio, chem and econ for AS. If you decide to do econ, drop bio. If you decide to do medicine, drop further maths. If you don't take maths, further maths, and maybe economics (not a requirement at most places but good to have prior knowledge of the subject), you will be at a disadvantage if you apply for economics. Another point is if you are still doing your GCSEs, try and aim for 9+ A*s. It will immensely help if you apply to LSE, UCL, Oxbridge (although less than the others) or Warwick for econ, and will also help at some med schools, such as Oxford and Birmingham.