Answer Base

With the permission of the owner. but is it necessary for that other car to itself be insured itself. there is nothing in my insurance documentation to say that this is necessary. (this is third party only insurance) if this is the case, then surely i could get cheap insurance on a mini and drive my friends ferrari even if my friend cannot himself afford to pay for insurance for his ferrari.

Yes you could do that but would your friend remain your friend if you pranged his car ? remember if you had an accident the only thing insured is the third parties property and person not your mates car. it is not actually the car that is insured but the person on the policy.