WD I bow to your superior knowledge - to be honest I can't remember why troops were used at Heathrow ... only that it happened.
There was a threat to down an airliner IIRC there was concern they had a MANPAD.
but to be honest I would imagine that they would be used to counter any threat before it became a issue or to provide specialist support to the Police AT teams.
Always in support of Police until requested to intervene or authority passed across.
The problem I feel is that we are talking about fanatics who don't seem to be fazed by the thought that sooner or later they are going to be in the sights of a Barratt and then likely to find out in fairly short order !
Broadly I agree but the two brothers in Paris seemed content enough to go on the run maybe that was live to spread havoc another day or weren't that convinced about paradise?