We have an unspoken rule on London Underground escalators: always stand on the right. Many a visitor to our city has been caught out by this rule as it's not made clear when you arrive, but if you stand in the way of a London commuter they'll soon let you know!
The left-hand side of the escalators is for walking down, and be aware some people run so you want to keep out of their way!
Generally Londoners are quite polite when they ask you to move to the right: you'll hear a loud 'excuse me' in your ear, which although it might shock you, it's better than other things they could say.
Remember to keep your luggage also to the right-hand side of the escalator as you don't want it to get knocked down.
Some of the escalators are incredibly long so get used to standing on the right and watching the commuters rush around, or you'll get too tired for your days sight-seeing if you try to keep up.