Wasn't the original idea to have sensors in gantries above the road which detect a box in the car? This is very similar to the peage in France, where you can buy a box which goes in the front of the car and is detected by the equipment at the peage which then raises the barrier and debits your account. I guess cars without a box would be photographed as with speed cameras or the congestion charge. GPS would not seem to be involved and every authority which introduces them will be expected to work to a common standard which will apply across England.