And why do they continue today?
I'm currently in London with a school group and someone mentioned that there are currently seven countries in the world that drive on the left side, and all used to be British territories. So why didn't other British territories conform to the British driving style?
First of all I must point out that Japan drives on the left side of the road and they were never colonized by Britain.
Most of Europe drove (or rather walked) on the left for security reasons as it was easier to draw a sword, knife or other weapon to attack an opponent if they were standing on your right- as would happen if you were walking on the left side of the road. In 1300 Pope Boniface VIII advised pilgrims to travel on the left side of the road for this very reason, and security is the same reason why the Japanese traditionally traveled on the left side of the road.
Napoleon encouraged traveling on the right hand side of the road in all the territories his country occupied (the greater part of europe was under napoleonic control at one point or another). Napoleon never managed to have a significant influence on the United Kingdom, and thus they kept the traditional laws surrounding travel
a popular question! check out this section of the FAQ*
*see the popular questions link on the sidebar or the wiki tab above