I submitted to about a dozen American literary agencies. I chose those who already represented British writers, especially of thrillers, or if they mentioned being interested in submissions from English language writers world-wide.
I didn't alter my writing sample to reflect American spelling or punctuation - not out of arrogance, but more on the basis that the editors have got to have something to do!
One difference that I did find, is that American agencies are much more prompt in replying than British. Some of this could be down to them deciding on the basis of where I lived and where my novel is set, of course, rather than any other factor. I'm not casting aspersions one way or the other, but I think that British agencies would be more open-minded to accepting stories set in the U.S.A. than the other way round, which is to your advantage.
So when it comes to getting a response from British agencies, expect a wait of several months. There are some agencies who I submitted to in January, who still haven't replied - and perhaps they never will.
The first thing you learn, is you've always got to wait...