Don't do it!
I am a third year mathematician at Oxford. It's horrible. I loved maths at school and was very good at it, but uni maths is so different. The course is very formal and boring. You get no choice in your first year and first half of your second year. Second half of second year you choose 10 options from 20, which isn't very much when there will be areas of maths you don't like (eg: stats, analysis) and then your third year is quite limited and dictated by the choices you made in your second year. Exams at the end of every year mean you will have had 6 summers of exams in a row and they're bloody hard. If you want to get a first you need to spend your life in the library, a 2:1 is manageable but not fun. With the choice again I would go to another uni and have more fun and still get a good degree. I hardly do any work as have lost all motivation, so will probably end up with 2:2, I've had fun but could have got an easy first at a different uni.
As much as I hate my course, I do love Oxford. It's very pretty, has a good atmosphere and the colleges give you good sets of friends to live and play with, and it is an Oxford degree at the end of the day.