I applied to (and got offers from) Cambridge, Imperial, Warwick, Bristol, Durham for Computer Science and Maths. I have a friend who independently applied to the same ones for straight Computer Science. I rank them in that order. For pure computer science I'd rate Bristol higher, I really liked their department when I visited. In the end I only chose Warwick as an insurance over them because of Warwick's reputation for maths. Also worth considering are UCL, Edinburgh, Manchester and I guess St Andrews.