UK agents - some differences
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 had a handful of partial/full requests from UK agents, I had a lot more success from US agents.
The submission process in the UK is often quite different (there is not as much emphasis on the query letter, but a huge importance is placed on the synopsis). Some points worth noting:
* Some agents do not accept simultaneous submissions, even at query stage. I think Maeve Binchy's agent is one of these. So if she is your dream agent, keep this in mind.
* A LOT of the big agencies only accept snail mail submission.
* My personal experience is that they are slower to respond.
* Many will only accept submissions from UK (and Irish) based writers.
Re. the query letter/cover letter: It varies from agency to agency whether they want a "cover letter" or a more American-style query letter. Generally this information is on their website (if they have one) or in the Writers & Artists Yearbook
http://www.writersandartists.co.uk/ (a must-purchase if you are submitting to the UK). If they don't mention it, a good rule of thumb is: electronic submission - query letter; snail mail submission - cover letter.
If it's a cover letter, it's less about the book, and more about you, the genesis of your idea and your background (kind of like an extended bio-section). If it's a query letter, it's the American-style query letter you are familiar with.
With a cover letter submission, a lot of weight is put on the synopsis (more so than American agents put on it). I have heard many UK agents comment that the cover letter is barely read, and the synopsis decides for them whether they go any further (read the submitted pages/request pages).
Also, with a cover letter submission, standard is a synopsis plus the first three chapters/first fifty pages, although this too varies from agency to agency.
If you have any questions, I am happy to help.
Dave