If you can eat it, you can Smoke It...
The rest is up to your taste. What part or the Beef animal is your local London Broil cut from? Here in PA, some stores label Flank Steak as LB. Some, a 2" Slab of Top Round sells as LB and other places, a thick cut of Chuck is LB. All get smoked at 225°F of higher. Flank is best no more than an Internal Temp (IT) Of 140 °, Medium, smoke to 120° then Sear to 130 with a 15 minute rest. I like more Rare and smoke 110° then sear to 120°F and rest. Finishes at 125-130, med/rare.
Round and Chuck are Tough hunks of meat. They can be smoked med/rare to medium but will take more Jawing to get them down unless cut Lunch Meat Thin. With these cuts, smoking to Probe Tender, a therm probe or skewer slides in with no resistance, at an IT of about 195° is a good way to go.
Marinades help to add flavor but do little to Tenderize. Below is my Go To for marinating London Broil or any Beef Steak. Marinate Overnight or longer...JJ
Grillin' BEEF Marinade
1C Soy Sauce
1C Brown Sugar
1/2C Minced Onion or 2T Onion Flakes*
1/2C Mirin or Wine (Any)
1ea Korean Pear, Delicious Apple, peeled and pureed or 1/2C Apple Sauce
2Tbs Minced Garlic
1Tbs Minced or Grated Ginger
1Tbs Sesame Oil
1Tbs Sriracha (Optional)
Combine all and Marinate 2 Flank or other Steaks, 24 to 48 hours. Makes great Jerky too. Up to 4 pounds of Beef Strips.
NOTE:* I remember Dad using Dry Onion Flakes, but Fresh Onion works too.