While correct for the UK you aren't thinking past that and assuming any other way is incorrect.

The reason driving on the right hand side of the road became popular in America is similar to left hand drive in the UK.

When the Americas were being colonised wagons and carriages were use extensively for shipping and transport. These had multiple horses to draw them. With the right hand being dominant it was benificial for the driver to be on the left of the wagon so his right arm was in the centre to control the animals. Because of this it was easier to see the road if you were on the right as it put the driver closer to the centre of the road. Therefore the roads and facilities were constructed to accommodate this.