Biology and Chemistry are a must, and so is maths, so those would be the three you take to full A Level, and physics is helpful, but if I were you I'd take those three and choose one you like, as you can always drop it after a year (If that's how your sixth form do it) or continue it and have one good subject that's a light relief, those three subjects are hard and it will be a lot of work (In the first week my school was setting 4 hour biology assignments!!) and it's nice to have one subject that lets you relax (For example I'm taking history, psychology, economics and photography and photography is a good way to de-stress as those others are hard work and lots of essays,)