How do gap years work?

I'm concerned about my predicted grades.
If I do better in my actual a-levels would it be better to take a gap year and apply with those? How does that work? Are you allowed to do that? (apply with your Yr13 original a-levels a year later or do you need to retake your actual a-levels again even if you did well etc...?)

Yes just reapply with your achieved grades. No need to retake if you already meet the requirements. Your grades don't expire!