Jim,
Here in Spain they are much more car nuts than the UK, they seem to want to touch the car all over.
I have put two small signs, one in the bottom of the front windscreen and one in the glass behind the drivers head. They say ( In Spanish of course) exactly what the car is, the HP the year etc and how many were made. Also the fact that the body is made of Aluminium that is just 1 mm thick so please do NOT touch.
Then is asks them to respect a car that is a collectors item.
Since I put them on I have no problems at all, in fact I have seen one or two people actually telling people off for not reading the notices and touching the car, whether this would work in the UK I do not know as the Spanish actually respect classics like these cars are.