The van you passed was a "safety camera" van. i.e. speed camera.
There are an increasing number of vans with NPR number plate recognition cameras in the back. All these are doing is checking for crims on the move, untaxed vehicles etc.
Sometimes there will be a little sign saying what they are doing, e.g. checking for tax.
I could launch into my usual tirade about the job creation schemes surround road tax collection and enforement when the govt could could just put on the price of fuel, but I won't
Ray