In my irresponsible late youth A friend towed me using his Ducati--I on my Norton Commando from Darlinton to Catterick on the A1M--we had researched this and using a tow rope was not only legal but the rear bike constituted a trailer.
The incredulous copper that pulled us over had to let us go when we drew this to his attention. Although nowadays they'd have got us for something
cheers
Barry