I'm doing maths at warwick.
First of all. Student(undergraduate) wise, cambirdge has the best mathematicians in the country and some of the best in the world. after that warwick,oxford, bath,ucl, imperial etc are all almost the same since they all require an AAB or AAA in a levels. Oxford's interview is not that difficult to cope for a moderate, not easily stressed person who likes the subject. Also no university other than cambridge requires STEP exam with excellent grades, an exam which distinguishes extreme ability and mathematical insight.
Techologically (buildings, labs etc) I think it's Warwick>Imperial>Cambridge>rest
Reputation wise the best one is again cambridge, with Trinity, it's best college in maths, having more fields Medalists than all oxford's colleges together. Then it's oxford due to the(still) world wide reputation and then imperial.
Now about the mathematics stuff of all the aformentioned universities it depends on what subject we're talking about. For example warwick is very strong in geometry (Differential, algebraic etc). Imperial is extremely good in mathematical physics (which in my oppinion is the future of physics). Cambridge is always good in more abstract areas, like group theory, fields, rings, number theory etc.