Look before you leap
My heart jumped a few beats when I instinctively looked left and saw no cars coming and put one foot on the road only to look to the right and see a truck (pardon me, a lorry) coming right at me from the right. It would have been a horrible way to die. No one would like their tombstone to read "Died looking the wrong way while crossing the road."
Fortunately in Central London, they had signs for pedestrians at crosswalks telling you to "look right." I wonder how many people died before they put those signs up.
Moral of the story: in a new country, always look both left and right several times before committing your foot to the road.