Warwick is generally very highly regarded for economics and mathmatics. There was a thread on here (if you search) that concluded...
This is from 2007
01 LSE
02 Oxford, Cambridge
[small gap]
04 Imperial
[big gap]
05 Warwick
06 UCL
[huge, huge gap]
07 KCL
08 Bristol
09 Nottingham
(An MSc from CASS after an OK undergrad
and you're about on par with 07-09)
[gap]
10 Bath
NOT MY LIST. you disagree? take it up with http://www.thestudentroom.co.uk/showthread.php?t=351393.
any redbrick uni or russel group (or such equivilent) will put you in good stead.
Having said that a bad degree (such as a fail) from a good uni isnt worth toffee... The course and uni that allows you to do the best you can is the way to go. Dont get too concerned about which uni is 'best'. I have a friend at birmingham for economics who loves it and chose it over other unis listed there. A happy student = a hardworking student!
I hope this materially answers what you was looking for, however dont underestimate open days! get a good feel for each uni!