depends on what cut it is... "london broil" is not a specific cut of meat, it is terminology for it's useage, the way it's cooked, which is by searing and broiling medium rare. You could smoke it but only raise it's internal temp to 135° - 145°, otherwise it would be dried out, and I would sear it on a grill first also. Cooking low and slow should be reserved for less tender, fattier cuts of meat that will tenderize while cooking and have enough internal fat and marbling to do so.