Best time to resit?

If one is taking a gap year, is it better for them to sit a single AS module which will hopefully bring their overall AS grade up, in year 13 or during their gap year.
If they sit the AS module during their gap year, how does the declaration of that
subject work?
E.g. Physics AS, do they should leave that pending or do they have to declare their old grade?

You d have to declare your old grade, I d do it in year 13.

I'm taking 3 A2s and physics A2 isn't one of them. So is it still worth improving that physics AS grade?