In the UK, our dear insurance industry are inventing more and more excuses to charge more, restrict privileges and pay out less.
Unfortunately, many policies now, and near all from 2010, will preclude the current 3rd party arrangement to allow you to drive a car not owned by you.
Special arrangements will be made for the motor trade and for test driving prior to purchase of a privately owned vehicle, but as I understand it, the latter entails you informing your insurance company of the test drivers driving licence and current insurance details prior to the test drive.