Towing Speed

[4wheel]

This morning we went up to the Strawberries and Steam Fair near Swindon -lovely day with plenty to see.However,on the way up the M4 there was a goodly breeze blowing and we passed a car and caravan doing about 65mph.Now I said to the good lady that I thought (comments please !)that he was travelling too fast with a caravan in tow.
Low and behold on the way back on the M4 this evening there was a caravan upside down on the centre reservation,all mangled and bent,no sign of the car.Not the same combo as it had just happened but what is the consensus of a safe towing speed ??