If the car being towed has all four wheels on the ground, it is still a car in law and so must have tax, insurance, mot, as well as licensed driver.
If the method of towing takes two wheels off the ground it becomes in law a trailer, as it "trails" the towing car, and cannot steer itself, this needs no mot, tax, insurance, as "trailers" don't.
I've towed a MG using a pole whilst my traffic cop mate sat next to me.
I've even been naughty and PUSHED using a tow pole a car around 15 miles, using the rear tow eye as a push eye, but that's defo not legal, even though I did drive past a plod shop whilst doing it!!
Of the rigid bars I've used the ones with the spring in the centre are well worth paying the extra for.
tabetha