Kramer & Ethel are Incorrect.
I don't answer many other subjects on the AB but as a motor insurance underwriter i can answer insurance questions confidently.
Firstly, the myth that you 'have' to be comprehensive to have DOC (Driving Other Cars) is exactly that, a myth. If you're 3rd party on your own insurance then you are paying a premium that has scored you for '3rd party risk' and is therefore similar risk in whatever car you happen to be driving. The likes of Tesco, Direct Line, Privilege, Churchill, Egg, Virgin.....etc will allow you to drive other cars on a 3rd pary only policy.
The car DOES NOT have to be insured in it's own right, this is another myth. The reason companies may not allow you to drive un-insured cars is that the moment you step out of the vehicle, the car is un-insured and on a public highway which can lead to points for the owner of the said vehicle.
You will always get different answers by different people depending on what their insurer has told them. Although i've pointed out some common mis-conceptions, the only correct answer is the one given to you by your insurer.
I hope this helps.