originally all aircraft were required to fly 250kts including heavies. After representations from a large carrier based at heathrow, it was decided that heavies that could not fly 250kts clean on departure would be allowed to fly at 280kts. Although this required them to request high speed - which was always allowed. The airline in question then pointed out that all of their 747 departures would be requesting this so it was decided that they would be allowed to do this without having to request it.....
Not sure I understand your comment not long now