Don't forget the private sector
The original article includes: "But there are also concerns about the tech's increasing presence in the private sector for car parks and traffic monitoring – which wouldn't be brought under a code."
One of the earliest things that Plod said when starting this surveillance plan was that he wanted all the ANPR from filling stations which they were using to control "drive away" fuel theft. Then the Highways Agency announced that "of course" they would be making the entirety of their speed/congestion monitoring camera network available, though I believe that they more recently rolled back a bit on that. Others on these forums have indicated that their (private company) ANPR output is required daily, and in very specific format. It therefore seems reasonable to assume that any vehicle facing camera will be reporting ANPR details to Plod, though not necessarily in real time.
It is therefore essential that any "Code of Practice" includes either all cameras, or at the very least includes all ANPR information supplied to or abstracted by Plod.