The rules in London are that you must stand on the right when taking an escalator, this allows those in a desperate hurry to run or walk briskly on the left.
Never, never stand on the left. Now how are you supposed to know that if you are a visitor? Well initially by the runners getting cross.
Recently some of the stations with large numbers of tourists have been given another clue. Blue footprints painted on the escalators. The British like to be subtle about these things.