That amused me as I strayed away from 11+ to see what I'd be worrying about in 7 years time. But seriously it is based on student satisfaction, not how the outside (or rather "inside") world views the maths courses.
My ranking (as a former undergrad maths and now working in academia) would be:
After those "top" I'd think of the other main institutions - Bath, Bristol, Manchester, Sheffield, Sussex, Lancaster (yes it does have a particularly well-renowned statistics group at least), Kent etc. etc. (that list is not exhaustive or ordered). You can probably gauge relative levels from the admissions offers.