How do gap years work?

I was previously predicted A*A*AA for my A-Levels, but I didn't do as well as I would've liked in my UCAS exams. My UCAS predicted grades are A*ABB, and my dream schools all require A*A*A or A*AA in top 3 grades. I'm only applying to A*AB and below universities this year, and hopefully I'll do better in my actual A-Levels. If I get the required grades in my actual A-Levels when would I apply to the A*A*A or A*AA universities? Also, would taking a gap year to apply with better grades decrease my chances of acceptance?

1) When applications open, in September I believe.
2) No, not all.