The actual meat behind the misleading label can be any of the tougher cuts such as Top Round Steak, Chuck Shoulder Steak, Flank Steak, Sirloin Tip, etc. Conscientious butchers really ought to be adding to the London Broil label a short line actually identifying the cut of meat.
The meat needs to marinated for anywhere from 8 to 24 hours. Preheat the grill or broiler (you can also do this on a barbeque or pan-fry it), and place the meat in a pan on a rack about 5 inches (13 cm) from the heat. Cook for 7 to 10 minutes per side. If you only like well-done beef, then don’t make a London Broil. The meat needs to end up somewhat rare in the middle, with the ends being about medium-rare. If you cook it any past that, you will have shoe leather and will have wasted a cut of meat that you could have braised or stewed instead. Remove from the heat, place on a board, and slice the meat thinly across the grain (which probably will mean slicing it width-wise rather than along the length.)
London Broil is not English at all, it is North American. The technique first appeared in print as “London Broil” in the US in 1931; James Beard thought that Philadelphia was the actual city of origin. It is true that originally Flank Steak was suggested as the meat to use.
Literature & Lore
Sometimes, you will see the term “London Broil” used to refer to a mixed grill platter containing various meats including lamb and pork.