i always thought i could drive another car 3rd party as long as its not owed by myself and is insured with the owner,

i used my friends car today which hasnt its own insurance but they use it on a traders policy, i phoned my insurance up to check if im insured on it and be covered on mine..
I was told that the car dont need to be insured in its own right and i can drive it without any on it as 3rd party..

when did they change that??