A couple of comments not directly related to the incident that started this thread - and I speak as an air traffic controller and not a professional pilot.
First, there is a distinction between being responsible for the aircraft and being in command of a situation in which the police or military services are involved. I'm not sure what bits of legislation give the powers but I think police powers trump most others. In the event that an aircraft commander was challenged over his or her actions regarding the aircraft and its safety, I would guess that it would be a generally acceptable defence if the actions were in compliance with police instructions.
As alex says, if the commander decides to evacuate the aircraft, he or she is the one who can do it - and it probably doesn't make much difference what the police say! But on the other hand, you'll probably find that the local police do have powers wherever your aircraft is parked up on the ground - and their powers will relate to far more than those of the commander in respect of the aircraft.
Taking off my bar room lawyers hat - on a related, and far more common issue, you might like to think about who is responsible for what when your aircraft is being pushed around by a tug. I recall reading about an incident at Heathrow a while back where this question was significant and I don't think anyone knew the answer. I did hear that the CAA was looking into these things but I don't know what he outcome is.
Both this and the situation with the police are rather grey areas!!!