When it comes to Russel Group Universities, Oxbridge is a bit in a league of it's own! A*AA is a impressive set of grades to get so it requires allot of hard work! When it comes to the Mathematics, with a B you can still get an A* (you need 90%+ UMS on core units and an average grade of A). With your UMS scores, although just a tad bellow 90 they still show the ability to achieve the aforementioned grades. However because Oxford is so prestigious and receives so many applications, you need to make yourself stand out. As well as getting a high score on the Thinking Skills Assesment I'm sure activities such as debating, sports (if you're into that), or other extra curricular activities (like in my college we had a stock market club) will make yourself look more appealing to an Oxford admissions tutor. Getting the grades, which for you seems realistic with hard work, is only the first hurdle. Take time in your A2 year to enrich your application with extra curricular activities as well as nailing your personal statement to really get your passion and skill in the subject across.