Make the sauce: Whisk all the ingredients for the horseradish sauce together until smooth and creamy. Put in the refrigerator for at least 4 hours to allow the flavors to meld. Serve in a ramekin beside the meat.
For the London Broil: To make the marinade, mix together all the ingredients except the meat. Place the meat in a shallow dish and cover with the marinade. Cover with plastic wrap and refrigerate for at least 4 hours but no more than 24 hours.
Remove the meat from the refrigerator and bring it to room temperature before cooking.
Preheat a grill to high, oil the grate, and set it 5 to 6 inches above the coals. (Alternatively, preheat the broiler to high and set a rack 5 to 6-inches below the heat source.)
Remove the meat from the marinade (discard the marinade) and pat it dry with paper towels. Grill on each side for 6 minutes for rare or 7 to 9 minutes for medium-rare. Transfer to a carving board and let stand for 8 to 10 minutes.
Slice the meat at a 45-degree angle across the grain and serve with the horseradish sauce.