Ace's overnight thread gives you plenty of British information, like why they still drive on the left side of the road.
Taking the left hand side in traffic is a habit that goes back hundreds of years, possibly as far as the ancient Greeks, Egyptians and Romans, but certainly to an era when people habitually carried swords when traveling. As around 85-90% of humans are right-handed, passing on the right-hand side would leave carriage and cart drivers more open to attack from people coming the other way. Knights with lances, squires with knives, peasants with pitchforks, everyone had to be ready for a dust-up at a moment’s notice, and that meant keeping to the left so you could get a good swing at your assailants. Granted, this did mean they were more vulnerable to be attacked from the pavement, but no system is entirely foolproof.