Even biology isn't essential for some universities. Chemistry, however, is. Think carefully about maths and chemistry though. If you want a challenge they're great, but if you don't like loads of complicated stuff they may not be for you.
Biology is needed for some medical schools, so if you don't take it it'll reduce your choice of med schools greatly.
Maths is not necessary at all, although many potential medical applicants take it anyway. Some colleges in Cambridge and I *think* Sheffield gives preference to those with three science/maths A-levels (so the third subject can be Physics, if needed).
Also bear in mind mot universities look at what else you do besides your academic route. Having maths does make chemistry easier, however it is completely possible to do chemistry without it.
i think chemistry is the vital one. Alot of unis gave requirments something along the lines of "chemistry 2 other sciences (including maths as a science)", so personally i would do something like chemistry, physics and maths to keep pretty much all the science options open at uni.
It's not essential, but another science such as Biology or Physics would certainly be beneficial - and I'd advise Maths. And then your fourth subject could be something completely different, like a language or an essay subject.