In the United States, the consensus seems to be that a London broil is usually a cut from the top round. But it can also sometimes refer to flank steak or any other cut that isn't from the loin region (where T-bones, Porterhouses, and filet mignon are from), but isn't large enough to call a roast (which are usually 4 pounds or more).
The cooking method "London broil" involves marinating flank steak and then grilling it or broiling it. The cooked steak is then thinly sliced across the grain so that it's easier to chew.
If you see a cut labeled "London broil" at the supermarket, it's a good candidate for this marinating and grilling cooking method. You could also use it for stews or smaller braises. London broils, flank steak, and top round steaks are all generally less expensive because they are tough and lean. On the plus side, they have a great beef flavor!
Are you familiar with London broil? How do you cook it?