Just checked my Admiral policy, and it states that the Policyholder Only can drive other vehicles;
The policyholder may also drive with the consent of the owner of a private motor car as long as the other car is not a hire or rental car, is not owned by you or your partner nor obtained by you or your partner under a hire purchase or lease agreement, within our territorial limits, providing there is a valid insurance policy in force for that car. The cover is Third party only.
Me and SWMBO are named drivers on eachothers policies, so irrelivent to us, but if we weren't, I wouldn't be able to drive her car, but I could drive my nextdooor neighbours car (with their permission).
The Insurance Company thinks it's being clever by doing this, but in-fact, we both save money (reduced permium) by adding eachother onto our own policies. I guess this would change somewhat if we didn't have impecible driving records!