I myself took a year out to improve my grades so that i could go to a better uni and all i can say is its a big risk getting to a different uni this way but its well worth it. I say a big risk because most universities tend to look down on you if you have taken a year out and will most definitely ask for much improved grades anyway.
I would advise against taking any extra subjects in your gap year as this will distract you from getting improved grades in your three most important a-levels.
Even if you do decide to go to uni this year, you might find some aspects of the course challenging but to be honest who wouldn't.
During this year i applied to three top unis and only managed to get one offer from king's college london and even they asked for AABE. So to sum everything up, concentrate on your main a-levels, keep motivated, and remember as long as you do improve your grades you will almost certainly have many maths courses available through clearing if all other options fail.