Didn't I see on another of your posts that you are doing an MA? If so, you'll find that postgrad life is a bit different than undergrad life. Do you mind if I ask how old you are? This will also make a difference. As you may have seen, UK undergrads are younger than US ones.
I came here as a 'mature' postgrad in 2000. I had just turned 30. I was placed in Uni accomomdation in a section specifically for postgrads and it also had a high international population and it was fantastic. I was one of the oldest, but it was still better than being with the first year undergrads!
I was at Reading Uni and they had an active SU and lots of organizations for various hobbies and groups (film, Latin - as in hispanic students -, walkers, American football, etc).
I think you'll find the UK students at the major Universities go home less. The smaller Unis, what we'd call "commuter" schools back home, tend to clear out. Try to find out the demographic of your chosen Unis.
Also ask your chosen dept specifically about social events, the mix of students, etc. My dept was and still is horrid at socializing and I HAD to find it elsewhere.