Can I just say to the people who think that mobilising troops to guard the airports and the 'postcard' sites think it is a cynical vote-winning ploy, I say you should get out more.
Having spent 8 years representing real people as a councillor, and nineteen years canvassing for people's electoral support, I can assure you that a cynical publicity stunt would be to make sure that there was a Bobby on every street corner. Not that it would have any effect upon crime rates, but that's what the public wants.
I know it's a very long time ago now but when, after September 11, police were deployed to the centre of London away from the residential areas and the provinces, the levels of petty crime shot up.
Whether it is a proportionate response to reliable intelligence is not for me to say. But I do know that robbery/burglary/car crime and anti-social behaviour is much more of a touchstone issue amongst the swing-voters of Middle England.