"Oh.. we'll just violate this part of the constitution once for the greater good. You don't mind do you? We need to observe these potential terrorists to keep ypu safe!"
I think the problem with your argument is here.
First of all, the UK doesn't have a "constitution," as you or I would understand it. But let's put that issue aside.
Your argument assumes, essentially, that people have some kind of fundamental right to
not be photographed while in public--and that CCTV cameras violate this right.
To the best of my knowledge, no such right exists, anywhere. Or ever has existed, anywhere.
Not only is there no such constitutional guarantee in Western countries--I'm not even sure that it's against the ordinary
laws of most countries to photograph people while they're out in public. Not only is the slope not slippery--there is no slope.
I know there's no such law here in Canada. I found that out when my teaching assistants' union went on strike. Some asshole drove his car right through our picket line, and then parked it nearby. I happened to have a camera, so I went over and took a picture of his car and license plate.
At that point, the asshole got out of his car, got in my face, and demanded that I hand over my film , claiming that I was violating the Criminal Code: he even quoted a numbered section. I told him that, to the best of my knowledge, I was acting perfectly within my rights, and if he thought I had committed a crime, then we could go down to the cop shop and let them sort it out. Eventually, he backed down.
I looked it up afterward. Not only could I not find a section of the Criminal Code dealing with photographing people in public--I couldn't find the section to which the asshole had referred. He just made it up.
When you're out in public, you're just that: out in public. People can watch you. People can listen to you. People can photograph you. If you don't want anyone observing you, then stay in your private residence, with the curtains closed.
That's where the limit lies.
Some kind of legal right to privacy while out in public would actually be an intolerable infringement on
other people's freedoms. It would essentially give you the right to tell other people where they can look, and where they can't; what they can photograph, and what they can't.
It would essentially give you the power of a king to command that his subjects avert their eyes, and not look directly at him. Now, where would
that power come from?