I think you are all pretty much on the nail about this being scare-mongering, BUT:
UKADGE has never been so weak in my opinion.
If you want protection from 9/11 type events, then you have to be prepared to down the airliner, and quickly. Scrambling fighters is not an option, unless you have them on permanent CAP, and even then, they will take time to reach the target. So what you need is a ground-based SAM system, with long range and fast ready time. But guess what? the Bloodhound was scrapped in 1993, the ulta-high power search radars? also scrapped, the passive radar linked defense system? also scrapped.
So what to we have left? Well, there are Type 91 and 93 radars at various points round the coast, fighters, and the short-range Rapier missile systems. Now, granted, the equipment is more modern than the old system, but I don't think it will do the job....
Oh, and UKADGE = UK AIr Defense Ground Environment
My credentials on this: 12 years in the RAF working at various radar and missile stations, including the Military Air Traffic Control Centre at West Drayton, London..
Just on heresay, I think the US is pretty much the same, apart from a few Patriot mobile launchers, there is little in the way of SAM defences....