Chemistry and Biology is a must for medicine, Maths is highly desirable too so these 3 will be your must-pick A-Levels.
I recommend doing 4 A-Levels only as doing 5/6 subjects will barely give you an advantage, it is the grades that matter the most instead of the quantity. As a forth, either Physics or Further Maths will be a good option as they are science-related but I personally think Physics is easier. Further Maths is a very vigorous subject and requires a lot of time and effort for most people.
In overall, I will do 4 A-Levels if I were you: Maths, Physics, Chemistry, Biology
In terms of extra-curricular activities, the non-medicine related ones do not count in your application so you just need to focus on the medicine-related ones.