I could look it up, but rather than do that I'll go on imagining that it all started when Henry Ford sent the first car over to England, and the recipient of this car - who happened to be the first British car driver - took it for a drive along the English countryside. As he drove on the right side of the road, he would grow increasingly agitated by always being caught behind horse drawn carriages - particularly at roundabouts, where horses would come to a crawl, as they never had a great turning radius. He decided to drive on the opposite side of the road, head on towards the horses and carriages. Luckily for him, though the fines for wreckless driving were high, people didn't quite yet know to look at his license plate to report him to the authorities.