There is also the question of where could the aircraft go given the decision had been made to land. I suspect a 747-400 en-route to Hong Kong is going to be well over its max landing weight for some time after take off and without dumping the fuel it may well have been four hours before the aircraft could land anyway so why not continue on to Heathrow? Should the situation escalate there are sutiable en-route diversion airfields, but why create an emergency situation when a realistic security assessment suggests there isn't one.
Tan - I think you miss the point. Company policy is company policy. The decision to offload the bag will almost certainly have been made by the company. Once that decision is made, the decision as to how soon and where that bag can be off-loaded is down to the flight crew. I would suggest that common sense prevailed in this incident. Four hours eastbound out of London there aren't that many places you'd want to take a 744 unless it really was a genuine emergency.