indaswamp,
London broil is NOT a specific cut of beef, but a method of preparing a cheap, tough cut of beef. Years ago, the preferred cut was flank steak, followed by top round. We've all seen flank escalate over time so anytime you see "London Broil" in the meat department, or at a butcher shop, it's probably, or no doubt, top round.
The typical preparation calls for first heavy scoring, or punching a lot of holes, in the meat then place in a acidic marinade overnight. The marinade generally calls for vinegar (white, red wine, etc), lemon juice, soy sauce, Worcestershire sauce, garlic, and olive oil. To finish, broil (or grill) at very high heat briefly until done to an IT of no more than 130°F. Slice, against the grain, very, very thin.
Obviously, you can use tri-tip, flank, skirt, sirloin, etc, with the same preparation and call it "London Broil" if you choose to do so. You'll just pay higher price for the meat.