<<How is one meant to tell a real police helicopter / aircraft from a fake one ? At any one moment in time, just how is Mr.Plod meant to know and address who is legitimately operating above the Capital from who is not ? Just how does one ‘close airspace’ ( other than with fighter aircraft ) ?>>
Strange comment from someone who cliaims to be an "airline pilot". As my buddy ATCO two has said, ATC has radar and knows who is there and who is not supposed to be there. Nothing moves over central London without some sort of ATC clearance. Areas within Controlled Airspace can easily be "closed" and it is fairly routine stuff around London for all sorts of reasons - security "incidents" and the Opening of Parliament to name but two. It's NOTAM'd and ATC prevent any aircraft from entering. That's why it would be extremely easy to spot an intruder. What happens thereafter would be up to the "authorities" but there is nobody in this world who can prevent another 9/11 over London - it just is not feasible, given the very short timescale in which such an incident could occur. Even if fighters could be scrambled in time (which I doubt) are they going to shoot down an aeroplane over central London?
As for Mr Plod.. Half the calls we used to get a Heathrow were from the police asking what a particular helicopter was doing over Buck House, or some similar high-profile place... you're right - the police helicopter.