You know, I was just about to post this very question, using those very names.
re: Nigel and Ian vs Julian and Duncan
Nigel and Ian just sound more British, as if they’re soccer hooligans or members of some late 1970s era progressive rock band. I’ve known American Simons, but I can’t seem to say the word “Nigel” in an accent that’s anything but pseudo-Cockney. “Hey, Nigel … bollocks!” “Hey, Ian … got a bloody Watneys?”
re: Last names as first names
The last-name-as-first-name trend is increasingly common among upper middle class American families.
Charlotte
Many American cities, towns and places are called “Charlotte.” I also have an Aunt Charlotte, and there were a few bluehairs at my old church that shared the name. Not British, but still old-fashioned sounding to my ear, like Ethel or Agnes.
Emma
AFAIK, increasingly common in the United States. For some reason, my lesbian acquaintences tell me there’s a disproportionately large number of “alternative” Emmas. I have no idea why.
Gaelic names
Based on personal observation, such names are increasingly common where there are large Irish-American communities; i.e. Boston, New York City, Buffalo. There seems to be some movement away from the likes of “Patrick” and “Catherine” into the more esoteric Gaelic names.
Some random thoughts
Italian-American names – are there any folks who aren’t Italian-American with names like Vito, Rocco, Vinnie, Guido, Angelo, and so on? Why do relatively few Italian–Americans give their children Anglo names?
Do any Brits have (American) Western place or themed names as their first name? It’s common west of the Mississippi – I’ve come across the likes of Dallas, Denver, Austin, Cheyenne, Houston, Sheridan, Taos and Kiva.
Are there any Beauregards north of the Mason-Dixon line?