I just noticed that the results of the Research Assessment Exercise for 2008 are out. For Computer Science and Informatics, the league table may be found here. Cambridge tops the chart with Imperial College, Edinburgh and Southampton coming joint second. (In reading the ratings, bear in mind that 4* rating is defined as ‘world leading’ and 3* as ‘internationally excellent’.) You might also want to look at the detailed interpretation of the RAE results offered by Edinburgh, according to which Edinburgh is by far the strongest department in the UK.
All the aforementioned departments host very strong TCS groups.
The ranking for Pure Mathematics sees Cambridge land in fourth place, behind Imperial, Warwick and Oxford. Is this surprising? Honestly, I do not know.
The debate on the RAE is raging in the UK, and I guess that it will go on for some time.