Most UK agents accept worldwide submissions. However, as many agents now switch over to accepting e-mail submissions (they are quite a bit behind the US: many agencies - and almost all of the old, established names - insist on postal submissions), I have noticed a trend where some agents are starting to specifically rule out US clients (only). Presumably this is to ward off a deluge of submissions, and their thinking may be that these writers have probably done the rounds in the US and haven't snagged an agent, so they are playing the numbers and assuming they won't miss that much great stuff, certainly not enough to put the hours into reading all the extra new submissions from US writers. It's only a small number of agents that have this policy at the moment, but this may grow in the future as more and more UK agents accept e-submissions and their workloads grow accordingly.
I am from Ireland (where there are maybe three literary agents, and one doesn't do fiction), so I started off submitting to the UK. An agent advised me that I may have better luck in the US as my "style is more American" (whatever that means), and because my story is set in South America. He was right. While I have a handful of partial/full requests from UK agents, I have a lot more from the US. I still submit to both, but its now tilted more to the US.
As a couple of other posters have mentioned, the submission process is often quite different (there is not as much emphasis on the query letter, but a huge importance is placed on the synopsis), and if you have any questions about that, fire away.