Oh my god, you are literally in the same situation I was in last year. I never really knew what I wanted to do with my life or what to study at uni. I got A*s in Maths and Further and I also prepared for STEP exams in my final year. I didn't enjoy those exams either and I never did maths for fun in my spare time, ever.
Maths was always my best subject and so I figured why not, and applied to study maths, thinking it would be easier as I was good at it. I am now a first year Maths student in Uni and I could not have been more wrong.
What teachers don't really stress enough about these days is how different uni is from school, sixth form, etc... Maths taught at GCSE and even A level is a matter of understanding how to solve a problem, using methods teachers teach you (basically make you memorise). Uni maths is understanding how these methods work, where do they come from? How would you prove them? It is not just a bunch of calculations, and honestly it is kinda stressing me out. I'm struggling already and thinking that this degree may not be for me yet due to my personality, I would not quit and leave uni (too stuborn for this to be an option
).
I am not saying this to put you off doing a Maths degree. You could have a completely different opinion about it once you start. The only advice that I could give you at this stage is to research other options. However keep in mind that though this degree is hard, no other degree is going to be piss easy. No matter what you apply for the level of difficulty will increase a lot, maybe being the reason why you won't enjoy it at first. I personally wouldn't take a gap year as in my eyes it is really a waste of a year. There are so many people at uni, even on my course who were on a different degree e.g civil engineering, biochemistry, etc... and have now swapped to do maths after their first year. You could always leave and apply somewhere else, nobody will stop you, and don't be afraid to do so. You have amazing grades so I'm sure you will be successful regardless of where you end up. Just focus on your exams for now and don't stress out about what you want to do in the future. I'm in uni and I still have absolutely no clue!
Good luck!!!