Fully comprehensive insurance just means your are covered in case you have damage to your car, an accident caused by you or someone else or a theft of or from your car.
It does not mean you are insured to drive anyone elses car, nor that anyone else is insured to drive yours.
Some policies offer 3rd party coverage driving other cars as apaid or complementary extra, but it would be written in your policy details, which everyone has.
So, unfortunately, not having read the policy details to know it wasnt included, is not going to be sufficient excuse for the police for being uninsured.
Drivers responsibility to insure he is insured, and your father in law could also be liable for lending the car to someone without proper insurance.