I was in the same position as you a few years ago and I'm now studying chemistry at Oxford, so it's definitely doable! I don't think you'd be at any disadvantage by applying during a gap year, as the tutors are really looking for your ability to take on new information and apply it to problems. If anything, the extra maturity you'll have from already having finished your A levels might give you a slight edge.
What advice are you looking for in terms of college choice? If you already have an idea of what sort of things you might be interested in doing for the Part II year (that would be quite forward thinking!), you could do worse than to apply to a college with a tutor who's researching that topic. Otherwise, the usual considerations of size, location, accommodation, etc. should guide you.
The maths you need at interview isn't particularly difficult and the maths for chemists paper is taught from scratch once you get here, so I don't think there's any particular need to worry about your maths going rusty. Certainly I didn't find it to be a problem.
I think you might struggle to find experience in a university research group, as you probably lack the lab skills and theoretical knowledge that they'd be looking for. I ended up working in a school lab, which wasn't particularly exciting but it did mean I got to try a few interesting experiments now and then, and also just kept me thinking about chemistry. But really, it doesn't matter what you do with your gap year.
I hope your exams go well in the summer. If you have any questions or want me to elaborate on anything, I'll try my best to help!