In fact, it's not an English item as far as anyone knows -- in England, 'London broil' doesn't actually mean anything. It's not a dish or a cut of meat either, but rather a type of cooking method ...
The name is thought to come from an attempt to make a leaner cut seem more appealing and high-class, which is a pretty genius marketing feat. But honestly, London broil can hold its own.
While originally made using the London Broil cut of beef, when that became hard to find, our family switched to using flank or skirt steak. In fact, I would argue my mom made it with flank steak ...
There's the London broil method, which involves marinating a lean cut of meat for a period of time before cooking it quickly (but never beyond medium, or it'll be too chewy). Some marinades ...