Legalities aside, I towed a Humber Ocean Pro 6.3 with a suzi 140 on a heavy Indispention trailer with my 1.6 Focus about 60 miles once. It recovered the boat no problem.
To be safe you should work on a gross trailer weight of 85% of the towing vehicle, ie if the car is 1000 kg, then you should only tow 850kg. Trust me that if you go any more, even only a few miles you will at best blow the clutch, and at worst strip the engine big end and gearbox. That does not even begin to take into account of stopping it.
