Traffic in England: why is it different?

One of the trademarks registered in England is how motorists drive there. The hand is the opposite here, with motorists driving on the left , and the cars also have wheels reversed , namely the right, while the upshift occurs with the left hand . But what is the reason?

This convention is not only followed in England, but also in all the United Kingdom, what means that Scotland, Northern Ireland and Wales do the same. Old England colonies, such as India, Australia, New Zealand and South Africa use the same kind of traffic. Japan also adopted it after the journey of an English minister to the country, who convinced them to adopt their transit system. 

But why the left side? Well, it seems that there is the beginning of the Middle Ages, when horse's movement was made ​​by the left hand, so that right hand could be free riders whipping their swords, in case of battle.