For most undergraduate courses which are strongly based in mathematics, you will need a good A level in mathematics (or the equivalent in other examination systems). Check with each university for particular grades as they can vary from A to C and some will also rely on the total points from your other A levels.
If your first language is not English, you would normally require IELTS score of 6.5 or higher.
Here is a selection of RSS accredited university courses entry requirements for UK residents taking A levels (updated in February 2011):
Oxford University, BSc Mathematics and Statistics: A*A*A, with the A*s in Mathematics and Further Mathematics if taken
Bristol University, BSc Mathematics and Statistics: AAA (A Mathematics, A Further Mathematics) or A*AA (A* Mathematics, plus at least one subject from Physics, Chemistry, Biology, Economics, Computer Science). If you have studied Further Mathematics to AS-level: AAA (A Mathematics, AA in any two A-level subjects), plus A in AS Further Mathematics
University College London, BSc Statistics: A*AA-AAA to include either A* in Mathematics or AA in Mathematics and Further Mathematics. A pass in a further subject at AS level or equivalent is required for all programmes.
You do not have to study a statistics module in your A levels but it will increase your chances of being accepted a place at university on a Statistics degree.
Further Mathematics is available as an A-level subject, and the equivalent also exists in some other systems. This allows you to study more mathematical topics for a broader and deeper understanding. However, all the necessary mathematics will be taught in the first year of university anyway, so do not worry if, like the majority of students, you have not studied further mathematics. Very few universities will require further mathematics in their entry requirements.
Whatever subjects you have done in addition to mathematics, universities are likely to require that you have done fairly well in them. Some universities will state a preference for other science-based subjects, such as physics, chemistry or biology. Other universities will not really mind what the other subjects are - provided you have done fairly well. This is yet again something that you must check in each university's publicity material.
Joint honors degrees
Join honors degrees such as economics and statistics, entry requirements are likely to be quite high, but mathematics may not be needed. Overall offers such as BBC in three subjects, or the equivalent in "UCAS points", are likely to be made.
The Open University specialises in catering for this market, but virtually all other universities will be happy to consider applications from mature students and will try to see whether the individual's skills and motivation are such that success on the course seems likely. Each case will be considered individually. If you are in this category, write to the Admissions Tutors for the courses in which you are interested with your c.v. and ask for an interview.