Try to think of the 'best' universities for maths in terms of tiers - where all the universities in the same tier are pretty much of the same level in terms of reputation/prestige.
It sort of goes like this:
Tier 1 - Cambridge, (maybe Oxford)
Tier 2 - Oxford, Imperial, Warwick
Tier 3 - UCL, Bristol, Durham, Bath
Tier 4 - Nottingham, Manchester, Kings, etc etc.