London Broil is EXTREMLY rugged.
a Proffesional chef taught me this trick to TENDERIZE it.
1. Trim all visible Fat.
2. Cut a 8-10oz portion of the meat in reaaaaally thin strips.
3. Soak it in Lemon Juice for 15-30 min prior to cooking. The acidity of the Lemon Juice actually cooks the meat somewhat, but it gets superrrrrrrrrrr tender.
4. Grill it on a REALLY hot grill or a couple of minutes on each side.