I hate to pour a bucket of cold water over this particular hysteria fest but I live in the centre of London and routinely carry at least one camera everywhere I go. I have never been stopped, interfered with, questioned or anything else.
Last year, in response to a similar thread, I took a walk through London photographing a number of high security locations, including the headquarters' of MI5 and MI6; 10 Downing Street (including the rear entrances); the Ministry of Defence; New Scotland Yard etc etc. I wasn't stopped or questioned, and actually the policemen at several locations smiled for the camera and posed for me.
The fact is that over the last 40 years there have been a number of serious terrorist attacks in London, some of which might have been foiled had not Londoners done what they often do, and looked the other way when people were behaving oddly or suspiciously.
And Olsen, as for the Met Police being one of the most corrupt police forces in Europe, you must be joking? I don't doubt there are a fair number of corrupt police in London but I can honestly say that none has ever asked me directly for a bribe, unlike policemen I've encountered in Brussels, Paris, Bordeaux, Prague, Athens, Venice, Krakow and Sofia.
Britain isn't perfect but it is nowhere near as bad as some of the frankly infantile posts in this thread are making out.