The bottom line is that someone had to make a decision on the facts available at the time, and I doubt that any of us know what information was available to that person. We seem to know about the note but not of any current background information that the police/security services might have possessed. For example, they might have had credible intelligence that some sort of attack was being planned somewhere. I'm not saying that they did - of course I don't know, but does anyone on here know either? If they do they won't be telling us and quite properly so.
The airport/airline authorities would almost certainly have acted on police advice who will always err towards caution because they have many lives to consider.
It's by no means unknown for terrorists to alert authorities to the presence of a planted device by means of a note or, more often, by a telephone call. They have many reasons for so doing.
If you're going to respond to a 'bomb call' you either do it properly or ignore it and who in their right mind would want authorities to ignore it? If doing it properly was an evacuation and diversion of flights then that was the judgement of the person charged with the decision: the person on the spot possessed of all the information available at the time. We armchair experts can criticise with the benefit of hindsight as much as we feel the need but we do so without having to bear a shred of responsibility.
Would some of you really have taken the risk of moving the aircraft with the chance of it exploding and causing numerous deaths and injury in a nearby terminal filled with incoming passengers whose flights continued to land?
The analogy of a London bus or the London Underground system is not a true parallel but I have no doubt that if it was felt circumstances warranted it then whatever was considered necessary, including a full closedown of the system, would be done. The same would apply at Heathrow if it was felt it was the proper action to take.
What no-one seems to have mentioned is the responsibility of the idiot or idiots who wrote what now appears to have been a hoax note. If he/she/they are caught they should be dealt with very severely by the courts. It undermines the fight against terrorism: every time a hoax call is made it takes up a huge amount of 'authority' time and sows the idea in the minds of the public that every call is a hoax. One day it won't be.