Well look at the claims in turn
He claims his device can read number plates. I don't know if that is true, but OCR does exist, and it is possible with current technology to do that. It could be true.
Lets look at the other claims:
It can detect speeding. Average speed cameras do exist that record number plates and the time taken to get from one camera to another. This claim could be true.
It can spot tax dodgers. If it was linked to DVLA's car tax database, then it could check that. Devices that do that are used. This claim could be true, but only in respect of car tax dodgers.
It can spot disabled people. Blue badges are issued to people, not cars. You can use it if the badge holder is in the car, or if you are going to pick up the badge holder. It is not possible for this claim to be true.
It can spot terrorists. There is no database of terrorists linked to registration numbers. MI5 and MI6 may have some terrorist-linked registration numbers on file, but there is no way you can make the claim that this devices will spot terrorists.
It can spot car theives and shop lifters. If a car is reported stolen, the registration number is forwarded to police and traffic wardens, and they will look out for it. In this respect, it might find stolen cars some of the time. There is no way it could find shop lifters.