If you don't meet your uni offer and they don't accept whatever grades you have achieved (based on teacher rankings) then I imagine you would have to wait a year, yes. The reason being that even if you sat the exam in October, you wouldn't get results for a while after. It wouldn't make sense to attend uni for a few months only to find out you didn't meet the offer.
If you do get into university, there's very little point doing the resits, as your exact A-level grades don't really matter much once you have a degree.