London Broil is not a cut of meat, although some supermarkets label it that way, but the preparation. The traditional cut used is flank steak or top round and needs to be cooked quickly, to rare or medium rare or it becomes very tough. Try to find a fairly thick cut of meat, about 1 1/2" thick. A cut this thick makes it easier to avoid over cooking and ending up with shoe leather.
Use a gallon size Ziploc bag to marinade the meat. Add 1 tablespoon yellow prepared mustard, 2 tablespoons red wine or red wine vinegar, 2 tablespoons lemon juice, 2 tablespoons soy sauce, and 2 tablespoons Worcestershire sauce. Close the bag and mix these well. Put the flank steak in the bag and remove as much air as possible. Refrigerate overnight. It's very important to do this in order to tenderize the meat. Don't skip the lemon juice or wine because the acidity of these helps the tenderization process.
To cook, remove the steak and wipe off the excess marinade. Heat your ovens broiler on high. Broil 1 to 2 inches from the heat for 6 minutes and turn, and broil for another 6 minutes. DO NOT OVER COOK!!!
Remove and let rest for 3 or 4 minutes to allow the juices to redistribute. Cut in very thin slices across the grain of the meat and serve. Delicious!