just follow this:
1) apply to oxford/Cambridge, Warwick, imperial, an insurance, and another uni of your choice
2)did you get an oxbridge offer? if yes ACCEPT THE OFFER
3) did you get rejected by oxbridge but got imperial or Warwick? then accept the one you prefer best based on location or other soft factors as there is not much difference between them
Oxbridge are unequivocally the strongest (with Cambridge being slightly better), however imp/warwick are the next best thing. these four unis have THE strongest mathematics departments in the country, final.
one thing to note about imperial, (what i have heard from friends there), the optional module choices arent that broad, whereas warwick has a larger variety. When choosing your courses you should always look at the modules to see if you have an interest in certain areas as all courses are unique.