London broil is not a specific cut of meat, but actually a method of preparation that involves marinating a beef roast, then grilling or broiling it and carving the meat into thin slices. You may find meat at your grocery store that is labeled as London broil which is likely a top round, flank steak or sirloin cut. Many cuts of beef labeled as London broil are not very tender by nature, which is why it is important to marinate the meat and avoid overcooking it to keep it as tender as possible.