First of all, you should contact the universities individually to see what they recommend given your specific circumstances. Generally universities are happy for you to take a gap year and apply with achieved grades, but some are less keen, particularly for maths heavy courses.
If you do decide to take a gap year, I'd recommend doing something in your gap year to keep your maths skills in particular up. In my case, I worked during my gap year, but I also tutored maths and sciences and self studied an additional maths module for further maths that my school hadn't covered.