I took a gap year between finishing my A levels and starting university because by the time I got my results, the academic year had already begun here, so I had to wait almost a whole year.
In my experience, taking a gap year is a very good idea. First, that's probably the only occasion when you'll truly be able to be free and idle in your life. A levels were tough, university is going to be tough. Why not take a long break in between? Plus, you get to visit places, read books, maybe work out, maybe re-consider which degree to study for, grow up, learn how to cook, become more mature, etc.
At the end of my gap year, just before leaving for university, I had to take academic books, and start studying to get back into shape academically speaking and not feel completely out of sync in class.
Some people in my class who didn't take a gap year, reached university still reeling from their exhausting A levels, and actually had to work harder than people who took a gap year like me. We were more relaxed and fresh, we were more mature, and more focussed on our studies (and less prone to feeling blue while we were getting used to living alone and bridging the gap between university and high school.)
So, no, I don't think the rust is permanent, a serious shape up starting one month before university should probably be enough to get in touch with academics again.