My sixth form offered me the chance to resit yr 12 with new courses because my Dad had just died.
However, I said no and I applied to a new college entirely to redo yr 12 because I needed the change of scene. I got in without having to explain my reasons, but I needed to take different courses because the government can only fund each course for one person once, not twice. Restarting yr 12 on the same course would be like taking that course twice.
So as far as experience goes, you can resit the entirety of yr 12 by either having particular circumstances in your existing college with new courses, or you apply to another college and apply for new courses.
BUT IMPORTANT (yes lots of caps) you cannot redo the same courses, even if its different exam boards. You have free education to an extent.