Do it before A-Level results day. I switched from Neuroscience to Art History and I only asked to switch about a week before my May exams. That was in 2006. Now I'm loving my course. Seriously, switch if you don't like your course. It's a complete nightmare being stuck in a course you hate/dislike. Get in touch with your tutor and with the dept you wanna switch over to and ask for an interview. Check out English dept's requirements - do you have their basic A-Level requirements? If you do, you're qualified to enter the course. What you got at the end of your first year would matter, because American Studies and English Studies are related. Given English's popularity, I'd imagine you want quite high marks in your first year.
Anyway, first thing's first - email the English dept, and second, email your tutor. They'll take you through the possibilities and details. It all depends if the English dept would give you an interview. When I switched, I'd already tried to sell myself via the emails I sent them, actually. Dunno whether it helped, but I suggest selling yourself in every way possible to get onto the course, as Eng is very competitive.