MPN11 - You may well be right about relaxed states of alert and obviously I dont know what systems they have. However I'd have thought that post 9/11, a country which is home to one of the tallest buildings in the world would always be nervous about an incoming unidentified target.
Ive been out of things for a very long time now, but I am sure in recent years I have read of Typhoons being scrambled to intercept unidentified civilian traffic approaching the UK which turned out to be entirely innocent.