Seen a van cruising around the North West with an anpr camera on each corner or the roof, clamping parked cars which must flag up on the database or something as untaxed, and then put a huge sticker on the windscreen says "this vehicle is untaxed". Cars seem to get lifted if they get left for a certain number of days. About time they started doing this imo, our house pays around £600 a year in road tax, why should other people get away with it?!
At the car show/steam rally they have near here every year, the police always have a stand with a few decent cars (everything from Mercs to Lamborghini's) on that have been confiscated for various reasons (no tax/no insurance/used in crime and the owners haven't got them back) with the dates for the auctions!
EDIT: That link above for reporting untaxed vehicles has always worked well for anything I've reported. The cars have either gone, or been taxed within a couple of days!