The fact that Britain doesn't actually use the metric system in everyday life appears to be a source of perpetual confusion to most Americans. I read an interview with David Schwimmer on The Onion AV Club once about the movie "Run, Fat Boy, Run" that he directed, which was filmed in Britain and starred Simon Pegg. The story is about an out of shape guy running a marathon to win his girlfriend back (or something equally inane). Anyway, a marathon is obviously 26 miles, and that's the figure quoted throughout the movie. The AV Club asked Schwimmer why he kept this nomenclature even though the movie was made in Britain, had British characters, starred British actors and was written for a British audience ("it's weird hearing British people say miles", they claimed). Schwimmer told them that they used miles because they knew the movie would cross over to America and they didn't want to confuse people.

Bear in mind that he must have spent several months in this country filming this thing, allegedly became friends with Pegg and must at some point have remarked on this apparently strange thing and had someone correct him. Oh and all our road signs are in miles.