Anyone who has traveled outside of North America to the United Kingdom or a Commonwealth country is well acquainted with driving on the left side of the road. Left-side driving stems from the past when automobiles were first invented.
In the United Kingdom, they have continued to drive on the left side, and their former colonies that are part of the Commonwealth have continued with this as well. When in doubt about which side to drive on, always look at traffic, and refer to this handy map.