London Broil with Soy Citrus Mayonnaise

Combine soy sauce, red wine, juices, 2 tablespoons oil, scallions, garlic, white pepper, and cayenne in a 1-quart heavy-duty sealable plastic bag. Add steak and seal bag, pressing out air. Turn bag to coat steak, then marinate, chilled, turning bag occasionally, at least 4 hours.
Transfer steak to a plate and pour 2 tablespoons marinade through a fine-mesh sieve into a 1-quart heavy saucepan, discarding remainder or reserving for another use (see cooks' note, below). Bring steak to room temperature, about 30 minutes.
Heat remaining tablespoon oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then cook steak, uncovered, until underside is browned, about 5 minutes. Turn steak over, then reduce heat to moderately low and cover skillet. Continue cooking until thermometer inserted horizontally 2 inches into center of steak registers 120°F, 10 to 12 minutes. Transfer to a cutting board and let stand at least 15 minutes (internal temperature of steak will rise to 130-135°F for medium-rare).
While steak cooks and stands, bring marinade in saucepan to a boil, then pour into a bowl and cool completely. Whisk in mayonnaise until combined well and chill, covered, until ready to serve.
Holding knife at a 45-degree angle, cut steak across the grain into very thin slices and transfer to a platter, then drizzle with any juices accumulated on cutting board. Serve at room temperature or slightly chilled, with mayonnaise.