Ok:
1) Chemistry is absolutely, ABSOLUTELY mandatory for any respectable medical school, you simply will not even be in the running, let alone considered, if you don't have Chemistry, whilst some universities don't have biology mandatory, Chemistry is required for all.
2) Medicine is a highly competitive field, they pick those with the heaviest academic subjects, good GCSEs and the lot, so even if you did take Bio and Chem, and say...History or Sociology, at AAA, may be passed over for someone who took Mathematics, and has AAA with Bio/Chem. In summary, to increase your chances, you should probably consider doing Maths too.