I think you have some misunderstanding about what the universities' mean by retaking after A2, they mean that they do not consider applicants who resit modules of a subject after the usual two years study of that A level subject. So if you want to take a different subject after you complete your AS exams, it should be fine!
I actually met a guy when i did my volunteering who was pretty much in the same situation as you!
I think he took Law, maths and something else (can't remember clearly, but he definitely didn't do chemistry) in his first year of college, then he realised that he wanted to do medicine after finishing AS, so he begged the head of his college to give him another two years at the college to complete the A-level subjects required by medical schools. (And he did really well in all the subjects, all A*s and A's)
And he emailed the unis about his situation before he applied last year, the unis said that although he has spent three years overall at college, the actual time he spent studying each A-level subject was still two years, he would be considered equally as other applicants. He then got interviews from both Imperial and Leeds, not sure what happened after that though since I stopped volunteering there...
So I would advise you to email directly to the universities you are interested in and ask them about your situation!! And you need to work harder to get A's in Maths, Chemistry and Biology as you will need at least 3 A's to get into a medical school in the UK!! Your school may not be willing to give you 3 A's for predicted grades if you only have Bs at AS.