Sheffield and Soton both have awesome CS departments.
Soton is definitely has one of the best departments in CS, Sheffield's a much busier city but a much nicer one than Soton which looks a bit of a **** hole.
Soton's labs are awesome though! And it has a very nerdy, but very very friendly CS community.
Sheffield i believe has one of the best graduate prospects for a student, perhaps to do with their student run CS business.
Have a look at them both, i really wouldn't opt for Sussex with Soton and Sheffield on the table
...we were on a break