What Avon & Somerset Police say in full is
"If the rope or chain is longer than 1.5m, it must be made clearly visible so as to assist other road users. A broken down vehicle is still classed as a motor vehicle and must be taxed,insured and have a valid MOT."
So, as I wrote above, towing with a rope or chain a non road legal car would be illegal. However, the Department for Transport states
"If you attach an A-frame to a car in order to tow it with a larger vehicle, the car plus A-frame counts as a trailer."
http://www.direct.gov...alasset/dg_200824.pdf
Trailers do not require Tax nor MOT and are covered by the towing-car insurance, however (as I also previously wrote) must conform to trailer regulations to be legal. Conforming to trailer regulations involves more than changing a registration plate and hooking up lights and brakes, but in principle can be done.
It is ironic that BRIGHT SPARK's assertion {only legal when two wheels are lifted - i.e. usage of a dolly} actually makes the tow illegal in two respects:
1. dollies may only be used for breakdown recovery to a place of safety, not transportation from A to B;
2. the towed car remains a car and so must be road legal as a car when travelling along the highway (Tax, MOT, Insurance etc.) just as it would need to be if on the end of a tow rope.
So if you lift two wheels of a non road legal car you have to lift all four!