Car tax must be paid on all vehicles registered in the UK, driven on or kept on a public road, and failure to do so risks a significant fine. We propose that any SORN vehicle should be able to be legally parked on the road, although we accept that it can not be driven on the road without tax.
You may be interested in these active petitions
It's not possible to tax a car that doesn't have a current MOT, so if a vehicle fails it's MOT and runs out of road tax, unless you have a private driveway, your options are to either sell the vehicle immediately, probably at a loss, or park it on the road and risk a fine, probably higher than the vehicle's value.
This mainly affects those on lower incomes, due to the fact that they may not have a driveway, own lower value cars and can't afford to immediately repair the vehicle.