I can't speak from experience here since I didn't do science A-levels, not even higher tier gcse for that matter, but anyone considering degrees in STEM subjects should be consider doing some revision over the summer before they start university. I'm on a science year 0 at the minute and have meet some people on the course who say they are surprised at how much they have forgotten, or people doing the likes of medicine who say they wish they did a foundation year because they fell like they needed the recap instead of being thrown into the deep end.
I wouldn't say not taking a gap year is a legitimate move based on what Iv said though, gap years are awesome if you do it right. Just make sure you are home around June and go over the stuff you did at A-level so you are ready to start the course.
Again, I' giving this advice second hand as I never did A-levels in sciences or maths.