Apologies if I'd got it wrong, but I thought it was based on GPS.
I've worked on some components for an Israeli system for fleet management, based on beacons and then transponders in the vehicles. Whichever way they choose to do it, all of these systems are based upon a vehicle owner wanting its system to be read (like a Dart Tag). Anything with an RF or Microwave element to it is easily blocked, so the only 'foolproof' system is optical recognition of number plates.
And thats a lot of cameras if they want to cover all roads. And crucially, a lot of cameras that 'they' have to pay for, as opposed to black boxes in the cars which I would guess we would have to pay for.
Back to my much earlier post, I reckon its all a big red herring to try and make people accept fuel duty hikes more readily.