Car dolly/ A frame towing laws...anyone know the score?

it seams a bit of a gray area.there's lots of contradicting info on line regarding them.some say as the A frame bolts direct to the car,[needing no second driver] it changes the cars cl to a trailer thus no tax/mot/insurance is needed..ive also read its illegal to ''transport'' a car with a A frame BUT its legal to recover a car with it?....can anyone shed a bit light on the subject?...thanks