I reckon I am like most folks in these parts (the US). We refer to a London Broil as a cut of meat when that is not strictly the case. It is a top round steak, although I have seen flank steak packaged as "London Broil".
It is a fairly thick and lean cut, most often braised, due to the fact that it can be fairly tough, especially if overcooked. In my opinion, grilling a top round cut to anything beyond a very light medium rare is a waste of time. The trick is in the slicing. It should be thinly sliced across the grain., as in S&P's photo. This shortens the fibers of the meat making it easier to chew.
Now, I ain't a butcher, and if my facts are out of order, I apologize. I defer to more knowledgeable members here.