To be honest, I think it depends on your age and if you look responsible,
If its a private sale then you should be fine and you can always say you wount buy without a test drive.
If its from a dealer then they should have the insurance to cover you.
What im saying is that you shouldnt have problems unless you drive a private sale car and prang it. Ive sold cars privately and allowed test drives, didnt ask for insurance as they guys looked old enough and responsible enough. If your a 17 year old wanting to thrash the b****cks off it then I can see problems, otherwise shouble be ok.
Disclaimer: I cant be blamed for anything that goes wrong