I think their close departure is due to their hub and spoke system at IAD. Since all the flights from dometic destinations arrive in a waive, then to keep the connection time shorter and competitive with other carriers, and to satisfy demand, they have more than one flight to LHR - long ago, I remember in ORD when UA would fly ORD-SEA and ORD-LAX with two flights only 1-2 minutes apart - they called it wing-tip flights.