Can I insure a car without it being registered to anyone?

I was driving a car belonging to a trader but not registered to him and got pulled over by the police and fined 300 pounds the car is in the impound but the car is not registered to the dealer how can I get the car back from the police impound I also have no insurance on that car because the dealer was repairing my car and I was driving his car and thought I was covered but I was not what can I do to get the car back please help