The history of the keep-left rule can be tracked back to ancient Greece, Egypt and Rome, and was more widely practised than right-side traffic.
Ancient Greeks, Egyptians and Romans adhered to the left side while marching their troops, for instance, if two men riding on horseback were to start a fight, each would edge toward the left, thus, they would be able to draw swords from their right and uphold a defensive position. Eventually, this turned into custom, and later, a law.
Great Britain remains one of more than 50 countries in the world that drive on the left hand side.
However, there is one street in London where you legally have to drive on the right (in fact, the only one in Britain) – Savoy Court in Embankment.
This street has been given a privilege by the Parliament to have traffic on the right because the of Savoy Theatre which is also on the right-hand side. Cabs can drop off their passengers directly outside the theatre without turning around and then are also well placed to pick up new fares from the Savoy Hotel as they leave the lane.