Try as hard as you can this year. If you want to apply for biomed in the meantime, in case you don't get AAA, then you could- but your main focus should be getting the highest grades possible.
Post grad medicine is much more competitive and expensive, so if there's any chance of you getting in as an undergrad then do it! Getting the grades, and applying on a gap year, is the most logical option.
Don't retake AS as a whole. Spending three years on your A-levels is a huge turn off to pretty much all med schools, and it won't be worth it.