Postgrad maths

Yes, A level don't matter in graduate admissions. But you will need more than a top 50 degree to get into a top 5 math department, especially if you mean top 5 in the world.

no just top 5 in the UK. Like what ranking uni would be the max to get into say, ucl or imperial. Doing maths at graduate level

By more than a top 50 degree I meant you would need high grades, strong references and the like. If you have those and the money and apply to at least 3 to 4 schools you can be reasonably certain to get an offer. This is for master's, a funded PhD is an entirely different ball game.

I have four points that I can't be bothered elaborating on since a quick search of the web (or even just this forum) would elucidate:

1. Ghost6 is chatting out of his hole
2. Aside from Part III at Cambridge, most UK Maths Masters programmes are relatively easy to get accepted to. As long as you have a fairly decent Maths degree from somewhere, you would be able to get into most places.
3. PhDs are slightly different - to get a funded place anywhere, you will need a good first and things like a good undergraduate dissertation and great references are also helpful/important. Whilst it is true that applicants from top universities may initially get more notice - as long as you are a strong candidate - it doesn't matter where you came from. The thing is, on the face of it - a first in the tripos from Cambridge says that you have a pretty decent background and ability at undergrad maths whereas a first at other departments who may have a less intensive course may not be as impressive. That said, you just have to contact potential supervisors and chat with them and let them see what you know and what you have accomplished.
4. You haven't even finished school - this discussion is way premature. If you are interested in doing maths at uni - try and get into a department that has a good feel to it and where you will prosper the most.