BSc (Hons) Observational Astronomy?

You'll need to get a PhD in astronomy to be an astronomer (or better yet, a PhD in physics) and that graduate program will require a lot of physics and math. You'll need to learn that in college. I suggest contacting a professor in the program and asking if the program will prepare you for graduate school, because there's not much you can do in astronomy with only a BS. If it won't, you could always apply to colleges in the US instead, where you can learn the math you need in college and then take whatever major you want.