Well your priority should be to get into a chemistry AS class asap, you've missed some work but not much at all considering the start is mostly GCSE overlap with some extras.
BUT if worse comes to worse comes to worst, some medical courses such as Peninsula and UEA don't require chemistry. UEA only requie an A level in biology and PMS only require one science. But having said that you might still be disadvantaged compared to other applicants.
It all depends on how much you want to do medicine but I'd strongly advise you to pick up chemistry, it's not late at all.