Why do English drive on the left?

"Of the five most popular Google questions about the English , 'Why do the English drive on the left?' is number one. [...] The most popular reason is that, in the Middle Ages, you drove on the left to keep your right arm free to wield your sword or lance at approaching traffic. In 1300, Pope Bonifacio VIII formalized the practice by telling all this pilgrims to stick to the left. While we clung on to the habit, as did most of our colonies, the French moved to the right after the Revolution. The left side of the road had previously been hogged by the aristocracy, forcing the peasantry to the right - a habit formalised after the Revolution".

How England Made the English by Harry Mount
Publisher: Penguin Books, 2013