From the Times Good University Guide:
Top 15 unis for maths (in order):
Cambridge
Oxford
Durham
Warwick
Imperial College
LSE
UCL
Bath
Bristol
Greenwich
Surrey
St Andrews
York
Nottingham
Leeds
Top 15 for computer science:
Cambridge
Oxford
Imperial College
York
Bristol
UCL
Southampton
Bath
Warwick
Surrey
St Andrews
Edinburgh
Durham
Swansea
Manchester
So I'd say your best bets would be:
Oxford (GG14)
Durham (CFG0)
Warwick (G1G4)
Imperial College (GG14 BSc, G4G1 MSci)
UCL (GG14 BSc, GGC4 MSci)
Bath (G4GD 3yr, G4GA 4yr)
Bristol (GG14 BSc, GG1K MSci)
Surrey (GG1K 3yr, GG14 4yr)
St Andrews (GG14)
York (GG41/GGK1/GG14/GG1K)