I'd advise not to revise full stop, and instead just take a break since you've just finished 2 years of solid GCSE. However, if you are to 'revise', don't revise A Levels. Instead brush over the fundamental GCSE stuff you've already learned.
Considering A Level courses such as Biology and Chemistry start off by refreshing students with basic GCSE stuff, it'd be a good idea, just to steady yourself and make induction less stressful; if you don't understand the basics, then you won't understand the actual A Level stuff. I remember when I started AS Chemistry, and I did an induction test which had all the basic GCSE stuff in, I couldn't remember any of it since I didn't brush up on it - it's fair to say I didn't do well!
If you try and revise the A Level stuff, then you'll end up looking at all the A Level stuff that you obviously won't understand, and this could make you confused or even put you off the course altogether. You don't want this before you've even started A Levels.