Posted by Johan (Keyholder)
(here)
Not only the Brits were riding (on horseback) on the left. It was the same in pretty much every country, till about the late 1700's.
When carriages started to replace horseback riding, the coachman kept the whip in his left hand and steered the horse(s) with his right hand (most were indeed righthanded, then too). Defense by the sword became less important. It was Napoleon who made driving on the right compulsory, except in the countries that he did not conquer (like Great Britain).
Another Napolean-related reason (to drive on the right side of the road): the French aristocracy drove on the left (before Napoleon). Napoleon did away with that, "the people" (and everybody therefore) had to drive on the right. So, it was to establish "equality". Driving on the right side was driving on the side of "the people".