If you can already excel in history then I doubt you'll find critical thinking much personal use. You will likely already know most of the course content intuitively (identifying flaws in arguments, constructing your own, etc.), so if your main interest in it is to develop your critical skills I expect you'd find it rather unhelpful.
However, it's worth noting that some universities still consider the full A-Level in applications. If you can take the full A-Level then it makes for some extra insurance if you're applying for a course that accepts it. I don't know how useful the AS alone would be, though.
Perhaps take a glance at a critical thinking revision site or textbook and see if any of the material interests or challenges you?