From what I've gathered from the past few years is that (for straight maths):
Cambridge and Oxford have very similar exams for the 1st year (this includes some questions being identical), and then in the 2nd/3rd years it's harder to compare. Most say (including tutors) that Cam is better for a masters because of the amount of options to choose from, not the quality (ie the course content/exam difficulty ect).
Warwick covers very similar stuff in the 1st year, but I think their exams are slightly easier. Warwick is more flexible than Oxford, (not sure how flexible cam is) so it's very hard to compare which is 'best'.
Now it's true that the very best students will apply to either one of Oxford or Cambridge, probably based on preference. So there will be lots of 'good' students at either Oxford or Cam. There will be lots of good students at warwick too, but it's probably true that most of the very best students will be at ox/cam
The uneducated individual will usually say cam>ox>war or cam=war>ox. This student is probably applying to university, and wants the university they are applying to (cam/war) to be better than oxford, or if they are applying to oxford, they will be hoping that it's an easier degree than cam. That's the general pattern anyway.
tl'dr , Cambridge=Oxford=Warwick for all practical purposes.
Now regarding actuarial science...I'm not sure.