Is the argument that Flock cameras are used for mass surveillance defensible, or just paranoia, and if it is real, does anyone have a good idea of whether the same argument would apply in the UK?
Our definitions of mass surveillance must differ for you to ask this. Flock cameras are marketed and purchases for mass surveillance expressly.
That doesn't mean the cameras are good; I think they aren't, or rather, at least in my metro, I know they aren't.
Flock will just start putting cameras up on private property and selling the data to the Federal government. Municipalities can do very little to stop this, and local governments are pretty poor at keeping their true reasons out of public forum deliberation. Loophole methods of prohibition ("Can't put up camera masts") are easily thwarted in court.