Vehicles now have to be continuously taxed or declared off-road (SORN).
If you allow a vehicle onto a public road without tax, or use it when it has been declared off-road you are committing an offence. The fact that it is being towed is not relevant.
Similarly to use a vehicle that needs an MoT certificate without one is also an offence. �Use� includes being towed.
These rules apply to all roads, including motorways.