You do have to stay in education until 18, so if you decide to do sixth form/college then you will have to take more. It is very unlikely that they'll let you take only one A-Level though, as you have to be in lessons for a certain number of hours per week for the school to get funding in order to teach you, and that number of hours is usually the same as 3 A-Levels worth of lessons. If the school doesn't get funding, they probably won't let you study there.