The wording is that if you are fully comprehensive, you can drive other vehicles "With the policyholders permission"
This means that the vehicle you intend to drive HAS to be insured in its own right.
Imagine the scenario of your car on a road, uninsured, that was unattended. You'd failed to set the handbrake and it rolled down the road and hit another vehicle, or worse, hit a person. Who's insurance pays for it?
Also, if you do drive a car that isn't insured in it's own right, and you get stopped, the vehicle will be seized, and will only be released on production of valid insurance for that vehicle. So if you're a long way from home, not only will the owner have to schlep it up there, they're going to have to sort out insurance too.
And if you take a risk on not getting stopped, don't go on the motorway, or any major roads, as most traffic cars and all motorway cars have ANPR cameras, which immediately flag up a vehicle which isn't insured and on the motor insurance database.