In a bowl whisk together marinade ingredients until combined well.
Put London broil in a large resealable plastic bag and pour marinade over it. Seal bag, pressing out excess air, and set in a shallow dish. Marinate meat, chilled, turning bag once or twice, overnight.
Grill meat, marinade discarded, on an oiled rack set about 4 inches over glowing coals, turning each once, 9 to 10 minutes on each side, or until it registers 135°F. to 140°F., on a meat thermometer for medium-rare meat. (Alternatively, meat may be broiled.) Transfer meat to a cutting board and let stand 10 minutes. Cut meat diagonally across the grain into thin slices.