Generally what universities like to see in maths students is that they've taken an interest in topics beyond the maths syllabus. In other subjects it's often easier to get involved in projects outside school, but since maths isn't a practical subject the best way to demonstrate your interest and ability is to research some more advanced topics and get into practice solving more difficult problems. It might sound a bit daunting at first, but if you use the right sources some of the complex topics can be more interesting than A level material without being inaccesible, and experience with tougher problems serves to both demonstrate an interest in maths, and help you if you need to tackle those kinds of problems in the future; in STEP papers, for example.