Try and improve on your English, if you can as it will help you understand your course better, you will learn English much better if you be around people who speak English.
Universities will take into consideration that you started to learn English a few years ago, so dont be too worried about that. Main thing is to always understand your work as you do the course, and if you dont understand anything just ask your teachers or ask on the student room, its advisable to get A grades at AS for chemistry and biology if you want to apply to medicine.
You will also need to sit an admissions test UKCAT if you want to do medicine at most universities and also BMAT if you apply to any one of the BMAT universities.
For your A levels, learn the knowledge then do practice papers to learn how to answer the questions, Your subjects arent a test of your English as much as they are a test of your understanding.