"The British custom of keeping to the left developed from jousting when competitors needed to keep their javelin or sword hand free to meet the oncoming horseman. As most people were right-handed this meant passing each other on the left.
The Continental custom of driving on the right was introduced by the Emperor Napoleon, who was left handed. Since it was he who established the first road system across most of Europe, right-hand drive was adopted on the Continent”
via Christopher Winn ~ I Never Knew That About London