London Broil, despite what you might find at the local meat market. is not a cut of beef but rather a method of cooking. It was one of the first recipes to become popular in early restaurants in the United States and so the name London Broil became synonymous with a cut of meat. Where this dish originated is unknown. It certainly didn't start in England, where the term "London Broil" has no meaning.
Originally London Broil was made with a flank steak, but over the years the name has been applied to almost any cut of beef that is very lean and less tender. Hence you might find London Broil being a steak or a roast that comes from the sirloin or round sections of the cow. This of course makes the whole thing very confusing.
To make matters worse the original method of the London Broil was simply a flank steak, pan fried to medium rare, cut cross grain and served. This method is perfect for a flank steak because it becomes very tough if cooked too long and by cutting it into strips you made it easy for even the dullest of teeth to get through.
Later the method was changed to include marinating the flank steak and then grilling or broiling it. This gives the name make a little more meaning. But here the origins get even more confusing. The marinade traditionally used for London Broil has ranged anywhere from a simple mixture of olive oil with salt and pepper to a wide collection of ingredients. You need to remember that chefs in earlier days tended to mix seasonings, sauces, and marinade more from what was on hand than from a specific recipe. To get a good marinade for London Broil try a mixture of soy sauce, olive oil, garlic, ginger, balsamic vinegar and honey. This gives it the basic flavors that makes beef great.
From here you need to grill the marinated flank steak, hot and fast and to no more than medium doneness. Overcooking will make it tough no matter how long it was marinated. When the steak is ready remove it from the grill, allow to rest for about 5 minutes, then carve, across the grain, and serve in strips. It's great on mashed potatoes (a traditionally favorite side dish). If you've been paying attention you will have noticed that most recipes that involve flank steak are prepared this way, from traditional fajitas to, well, anything with flank steak. This is generally a tough cut of meat, but it has great flavor and if prepared it right, people will love it.
Okay, now for all those other things called London Broil. These cuts all have something in common, they are lean and tend to be tougher, so the same rules apply. You might find "London Broil" in anything from a 1 inch cut to a 4 inch roast. Marinate for 2 to 3 hours per inch and grill to no more than medium. On the thick roasts you will want to grill it directly for about 2 to 4 minutes per side then indirectly for about 30 minutes. The internal temperature should not pass 130 degrees F. Allow thinner cuts to rest for about 5 minutes and whole roasts to rest for 10 minutes. Resting allows the meat to relax and the juices to flow back. Carve the London Broil cross grain and serve. It's a great way to get a really good meal out of a less expensive cut of meat.