hi, you will need at least 3 with biology, chemistry and maths as compulsory subjects. however, most unis in the uk will only accept students with 4 full a level subjects. my collegemates who are planning on reading biology normally take bio,chem, phy and maths
Most people do 3 full a levels and 4 AS levels. Chemistry and Biology are generally required for medicine and another science subject is sometimes preferred by universities.
Usually you do 4AS levels and drop one so you do 3 full a levels. Chemistry is an actual requirement and with that I would do biology and maths (maths isn't necessary though but will compliment and help). For your 4th you could pick either something you enjoy or a subject that shows that you can do essay based work as well such as english literature, a language, psychology or philosophy and ethics e.t.c