AS-levels are being 'decoupled' from A-Levels, and not all of them at the same time. What this means is that you can study AS-Levels, but that they will no longer count towards an A-Level - they will a qualification in themselves. In the future you will have to decide the level of study at the start of sixth-form: AS or A-Level. There will be no more ability to start at AS and keep subjects on at A2.
What you need to do is find the specification of the subject that you wish to study and figure out when the changes will be applied. Then you will know the situation with doing the AS-Level in it.