I think a lot of students have a grave misunderstanding when it comes to subjective difficulty from higher/lower ranked universities. This seems even more apparent in science subjects like mathematics.
Yes, Imperial's course or Warwick's course or Cambridge's will be difficult. No, it won't be stupendously easier somewhere else; there will still be a challenge. The courses at the top 5-6 universities only vary with respect to the amount that is taught in a smaller time frame. For example, Imperial might have introduced what I learnt in second year in their first year... but this doesn't mean that I won't have to do it sooner or later just because I'm at a lower ranked university.
Students at the top 5 are expected to absorb quicker, retain for longer and adapt more smoothly than their lower ranked counterparts, and while I agree that this makes their courses harder, it does not necessarily mean that you will find a 'lower ranked' university easier. (In the same way, it also doesn't mean that a person who got 90% at Bolton could not hack a masters at, say, LSE).
All you can really say conclusively is that you find the course difficult. You can have a go at it from a lower institution, but as others have said:
- Maths at university is miles apart from A levels. The first year is there to ease you in, and then by second term in almost every uni you get into the real stuff - analysis, calculus etc. It's not the spoon fed rubbish that depends on how good your teachers are.
- Just because your friend at Birmingham is doing well on the course now, at the beginning, means nothing. Nothing at all. There are plenty of people who get high firsts in first year because the modules are qualitatively no different to what they'd learnt at A level (with the help of good teachers and possibly extra tuition). These same people, when faced with the prospect of learning material from scratch and having to learn it themselves, failed. There are at least 5 people who fit this description in my year who are on a borderline third/2:2 grade.
- You are being extremely arrogant and overconfident of your abilities, and you're underestimating exactly how large the leap is between an A* at A level and a 2:1 at university.