Ye i will be studing nat sci at cambridge( I have just had a cheeky look at some of the stuff). Don't stress, if you needed FM it would be a requirement they have 2 'sets' that take the same exams but go at a different pace. Mathematics course B being for Further Mathematicians and course A being for people who just did A level Maths(as you suggested you will be doing this) . So you will be in the same position as all the people in Maths A and I'm sure Cambridge would have made it a requirement to have FM if it was that difficult without it.
Which college btw?