I have noticed that there were a number of vehicles clamped in the village where I work. They were DVLA clamps with notices put under wipers. I've been told that the DVLA automatically knows if someone has not paid road tax for a car. Therefore, any vehicle on the public road becomes illegal. If a car owner declares SORN to the DVLA and keeps the vehicle off road - ie on private land, then they are still legal until putting the vehicle on the road.
I noticed this car clamped for a few days, but the owner may not have paid the tax and removal costs. After a certain time, it gets removed by the authorities and sold at auction to recover costs.
No getting away from it in this day and age. Pay your car tax or loose your car. It's harsh medicine, but there it goes.