Yes and no. If your grocer has labeled the cut of meat you have as London broil, skip to the next paragraph. To purists, London broil is a preparation, not a cut of meat. It is a steak that is pan-fried (and therefore not grilled) over high heat only to medium rare, beyond which point it becomes inedibly tough. It is a useful preparation for lean, tough cuts of meat, such as flank steak, shoulder, and round. To the less pure, London broil is a preparation that involves marinating a flank steak, then grilling, broiling, or pan-frying it.
Some absolutely promiscuous butchers label various cuts of meat London broil, including flank steak, sirloin tip, top round, etc. We’ll assume you have a flank steak. For best results, marinate the meat for several hours or overnight in the refrigerator in the marinade of your choosing. It is easiest to put the marinade and meat in a sealable plastic bag. Turn the bag over once or twice. When you are ready, grill the steak over high heat for about 3 minutes per side, then turn the grill down to medium if using gas or move the steak to a cooler area if using charcoal, and cook for another five to six minutes, turning it once midway.
Finally, for a flank steak, the grilling is less of an issue than the cutting. Flank steaks are tough, so you slice them thinly on a diagonal across the grain to sever the tough fibers and make the flavorful steak chewable.