Lean cuts of beef, like top round and flank steak (London Broil), are not worth grilling, and should not be grilled, without marinating them overnight first. Cooks say 10 hours at bare minimum. Otherwise, these dense, wood-like cuts are like chewing gum when grilled rare, and like leather when overcooked.
In other words, something you could easily choke on, requiring an annoying Heimlich Maneuver.
They have to be marinated in enough acid - vinegar or citrus -to loosen up the meat. Some alcohol - wine or beer - helps in addition to the vinegar. Many people seem to like to simply marinate these cuts overnight in pre-mixed Italian dressing. (I use plastic garbage bags for marinating things in.)
Even with a good 20 hours of marinating, London Broil needs to be sliced thin after grilling.
Here's an assortment of London Broil marinades.
(For the barbie, I prefer wood to charcoal, and charcoal to propane. However, I use all three depending on what I am doing.)