This is from Wiki:
"
Many American butchers will label a cut of meat "London broil". This is confusing as the term does not refer to a specific cut of meat, but a method of preparation and cookery. The cut of meat traditionally used is
flank steak, but in recent years butchers have erroneously labeled
top round steak/roast as London Broil. Because the muscle fibers run the entire length of this cut, the meat can be tough if not
tenderized via pounding or massaging. Scoring, stabbing, cutting, penetrating, or otherwise mutilating the cut before sending it into the broiler results in a tougher finished product as it allows all the desirable juices to run out of the meat into the pan."
So maybe the O.P. got him self a tough piece of meat.