This is not restricted to England. Many post colonial countries “drive on the left”. Some countries that have no connection to English colonialism, like Japan, also drive on the left. And, yes to answer your question, we walk on the left too. I have heard many people over the years try to give explanations for this, however none have been more persuasive than the correlation to the automobile. It would be great if someone could cite sources from traffic law or the like with dates.
The point about escalators is actually another matter. It has come to my attention that when escalators were marketed and sold here in Japan, the major cities of Osaka and Tokyo purchased from both a British and American manufacturer, respectively. That somehow explains why the phenomenon that johnpowell notes about the London tube exists in Osaka, while the people in Tokyo (only 550kms or 340 miles away!) stand and walk completely oppositely!