747s have been regular visitors to BHX for nearly 3 decades, the dearth of the type in recent years has more to do with the economics of operating big twins than an unwillingness of operators to put them through BHX.
My only theory is the runway. When PIA began, the 747 proved quite popular but it couldn't fly direct (I doubt it would have been able to do so with half the load) so they had to fly via Copenhagen. Hellenic managed fine due to the fact they were only flying to Athens but load factors were poor. Maybe at some point they'll manage a regular 747 service.