If you are over 25 and you insure your car fully comp, most insurance companys allow you to drive any other car third party. You may not even know they have provided this as it just appears on your certificate. However if you are connected to the motor trade they automatically take this away from you.
Trade insurance is avalible for young drivers usually from 21-
Pricey though and you do need to be a business.
Insurance brokers are your best bet, they can usually help if they can get a sale out of it.