London Broil is a staple in my house due to it being an inexpensive and easy to prepare cut.
There are many many ways to prepare them but here's the method I use the most.
Start by tenderizing the meat, use either a jaccard or for this case I often use a paring knife and slice about 1/8 inch at diagonals both ways, creating "cross hatch" slices. I like the way as the meat cooks they open up.
I then marinate the meat in a combination of italian dressing, catalina dressing, worschershire sauce, garlic and dijon mustard. There are no measurements here I do it all to taste and enough to cover the meat. But the base is the two salad dressings, the rest are flavorings. Cover the meat and let refigerate.
Cook over a medium heat, higher and you'll get flareups from the marinade which I keep brushing on throughout cooking. I place it on the grill at and angle to match the slices I put in earlier, turning and flipping to match. It will moth take long to cook depending on the thickness of your cut. YOU DO NOT WANT TO OVERCOOK THIS! You're shooting for rare to medium rare here, otherwise chewing becomes very difficult.
Slice thin cuttting at a 45 degree angle across the grain of the meat.
Wonderful for sandwiches the next day.
Good Luck and Enjoy!