The term, London Broil, is a method of cookery, not necessarily a specific cut. it originally was a method of cookery for the Flank Steak; cooking it fast to medium rare, then sliced across the grain on a bias (slant). But, over time, it got over-used in its description to include almost any cut ".....for London Broil". Here is Wikipedia's definition:
But, any cut that is thick can be used; i.e. top round for London broil, bottom round for London broil, eye round for London Broil (split lengthwise), beef shoulder roast for London broil, arm shoulder roast for London broil, chuck roast for London broil, and so on. The primary method is to marinade the cut in an acid based marinade to loosen the meat fibers from each other for several hours, cook it medium rare to rare at a high temp, grilling the outside but barely cooking the inside. Then, slice across the grain (so the meat fibers are short) and, if possible, at a bias (widening the slices), making a normally tougher piece of meat more tender via both cooking method and carving method.