The first escalator was installed at Earl's Court in 1911. A one-legged man, "Bumper" Harris, was employed to use it to demonstrate its safety. Unlike modern "comb" escalators, the original "shunt" mechanism ended with a diagonal so the stairway finished sooner for the right foot than for the left.
Anyone not wishing to walk on the escalator was therefore asked to stand to the right to allow others to pass, leading to Britain's unique flouting of escalator etiquette which dictates in most countries that escalators tend to match the rules of the road.
SOURCE: David Long 'The Little Book of London Underground'