The simple fact is, it is another tier available to the PM on the day, and that persons call whereever it is in the world.
Special Forces train for these eventualities, but take time to muster, even if they are on standby as the QRF.
It is then that the local Police "Specialists" who in so many countries now, are heavily equipped, and rightly so, as they fill the gap, or the wait for the Military option should it be needed.
Clearly NIMROD was an overviewed piece of drama, and too much access by the media, where believe me, many things went wrong. However, in ALL these types of operations, you have to have a fair modicum of Luck, and lady luck was on our side that day!!