As Spy Blog says:
There is no evidence that any Islamic extremist or Irish terrorists or Animal Rights extremists or neo-Nazi extremists, who have exploded, or tried to explode bombs, or set off incendiary devices, have been deterred from doing so by the presence of CCTV cameras. Some may have been tracked down partially through the help of CCTV footage, after their attacks or attempted attacks, but that is not what this poster is implying.
There is no evidence that any of them who have actually had access to any explosives, have ever been caught in the act of "terrorist reconnaissance" of CCTV cameras, neither by members of the public (which is what this poster misleadingly claims), nor by regular Police street patrols, nor even by any covert surveillance of known suspects.
Since you do not need any equipment to check out where public CCTV cameras are, just your eyes and your memory, it is unlikely that any real terrorism or criminal reconnaissance of CCTV camera systems will ever be detected in the way that this poster implies.
This poster is just Climate of Fear propaganda, and it will no doubt be used to justify the harassment of photographers taking photos, perfectly legally in public places, which have been infested with CCTV spy cameras, something for which there is plenty of evidence for.
I lived in London for a couple of years in the 1980s, working for some of the time in the big department stores at the height of an IRA bombing campaign. When there was a bomb warning - and they were almost daily events - we each searched our own little part of the building and then carried on with business as usual.
He writes about the popularity of the Keep Calm and Carry On poster and ends the article as follows:
Dr Lesley Prince, who lectures in social psychology at Birmingham University, is blunter still. "It is a quiet, calm, authoritative, no-bullshit voice of reason," he says.
"It's not about British stiff upper lip, really. The point is that people have been sold a lie since the 1970s. They were promised the earth and now they're worried about everything - their jobs, their homes, their bank, their money, their pension. This is saying, look, somebody out there knows what's going on, and it'll be all right".
Poster from Keep Calm and Carry On.