This really depends on the agent’s own preferences, so try researching them as much as you can. I know this isn’t always easy. The younger ones tend to be rather more available online than the older.
I was always keen to represent non-UK authors, and for quite some time I had more Americans on my client list than Brits. This included a NBA shortlister, the Enron book that CBS made into a great movie, and Senator Orrin Hatch’s autobiography... and many more besides.
One thing you need to understand is that the conventional publishing business is pretty face-to-face. While in theory you could be represented by an agent living in Montana, in practice, you want someone who can actually knock on doors and take meetings with publishers. I used to be in New York at least a week a month for this reason.
As far as national preferences are concerned, formatting counts pretty low on my own priorities. I’m looking for an amazing writer / manuscript... the formatting and spelling can be fixed later. But I know some people are more anal about it than me.