Well anyone who does real risk assessment would quickly work out that several hundred body bags is actually a good return for wasting £1.7B from the airlines and almost certainly that much again from the general population.
Even with the inflated cost of life in my industry that would be a sensible 'justified spend' - the lives not the money!
There is some interesting work published on the effects of 9/11 on air travel. At least 1200 additional road deaths can be attritubed to people declining to fly after the event and instead dying on the roads whilst undertaking travel.
If NATS really has some risk assessment expertise is is hard to find much evidence of it!