If the only way to check if a car has been taxed, is insured and has an MOT is ANPR just where are the readers?
Very occasionally we see a camera van on the A36 betwen Salisbury and Southampton. I know Wiltshire Police traffic cars all have the ability to read and check, but again there are only a handfull to cover the county and to be honest they have more important priorities.
If you look carefully at the entry to most towns, you will see a blue pole with a small camera on the top. Not the green ones with a square faced camera, those are TrafficMaster.
The Police will automatically get informed if a vehicle of interest passes one of these. They are also used to automatically generate letters to remind you to get a tax disc.