lol, I was going to say "nearly all Americans and Canadians think the UK uses KPH", but one already got there.
No, you're wrong, the UK has always used miles and mph, that's what comes from your country's proud tradition of being completely ignorant of foreign cultures and countries and continuing stereotypes even when they're crap just because it's what Americans expect.
So in American films most of the British actors have stupid accents, like they're Oxford educated toffs or 1940s Etonian schoolboys.
Apparently when Americans hear a normal midlands/SE British accent they think the person is Australian, which I have personal experience of, so to avoid confusing the audience they have to put on a fake accent.
I think I remember seeing some shit film, National Lampoons European Vacation I think where in the UK part they made a joke about using Km/H instead of MPH, and in The Simpsons too I think where they made the same joke (probably ripped it off). So I'm guessing that's where it comes from.