I'm a third year Maths + Philosophy student. This consists of studying the same Maths syllabus in first year, and about half of it in second year with just a few modules in third year.
The first thing to note that is that how you do in A-level, STEP in my opinion barely affects how well you will do at the degree since the style is fundamentally different. I mean this more from the perspective that doing well in A-level or STEP is not a sufficient condition for doing at University Maths.
I've done a lot of modules in other departments (Stats, Econ, CompSci, Languages, Philosophy, Maths) so it might be more useful if I can give a comparison. Maths really catches up on you if you don't study it enough. By this, I mean that you can more feasibly go into other departments learn their modules and do reasonably well without doing their prerequisites. The individual difficulties into doing so differ, but they are at least approachable. The same can't really be said about Maths, aside from certain modules such as set theory which has few prerequisites. The difficulty of Maths is not at all about the exams. The exams themselves are often quite standard and don't require any new ideas or methods. The difficulty is more about understanding and building on those concepts.
At the same time, I don't think Maths should be hyped in terms of this difficulty. Once you have passed the conceptual barrier its comparitively tolerable. You don't need any supernatural insight (at least for Warwick Maths Exams) to get firsts in the subject. Comparing this with essay writing in Philosophy or other such subjects is difficult since its a different type of requirement to do well.
I agree with the 50/50 notion described earlier. What happens is that some students become completely disillusioned by Maths, especially for those in joint degrees. Other students kind of 'see the light' and end up doing very well. Its very much a sort of snowball in my opinion.