As long as you're dedicated you should be fine with a B. I got a B in chem through self teaching (I was home ed) and shortly became top of my class at A-level. Especially if you get A/A* on your own c: Although I would be wary, if you can drop to a B at GCSE just because stress, that's only going to increase at A-Level :| And A2's a b*tch.
I advise trying it for the first couple of weeks and seeing how you get on, my college let you switch if it wasn't too far into the term.