"I though I was covered" - this happens all too often, why is it such and ordeal to check? Why do people still hold this ridiculous urban myth that fully comp seems to make driving without insurance ok? If a driver who does not own the car is to drive it he must have insurance that says on the certificate, words to the effect:
"may drive other vehicles not for hire or reward, not belonging to the policy holder third party only" if it doesn't say that then he/she is not insured, end of story.
The beak/plod have heard all the excuses under the sun, there is no mitigation here.