With the millions of tourists traveling to London every year the transport system and public areas can get pretty crowded. To keep in good with the locals and allow people to get where they need to go in London stand to the right of the escalators. This allows a free space on the left for those keen enough or wanting to to get past and on their way.
It is not a matter of filling up every step as full as possible as some of the escalators in London, especially on the Tube can be quite long and you won't be popular if you can't get out of the way of the busy local trying to get up faster.
