When your dad took out the policy, there would almost certainly have been a question about 'Are you the owner / registered keeper of the vehicle '

I suggest your father looks at the insurance paperwork, to see if there was, and what his answer was. (just checked on my policy paperwork, my own 'vehicle details' section asks if I am the registered keeper / registered owner).

Basically, if you are the keeper, but only a named driver on the policy, then the insurance company may regard it as 'fronting'. Which is viewed as attempted fraud.

I think that should be 'the insurance company will regard that as fronting'. Sounds like its either being taken to court with all those consequences (hefty fine [maybe worse] and still the owner paying for the cost of a new car] or at best the owner paying for the cost of a new car if the insurance firm takes pity on them (?) and doesn't tell Plod. Either way, they'll probably have their insurance cancelled and find it difficult (both father and son) to get insurance again and/or at a reasonable price.

They don't take kindly to fronting as it diddles them out of thousands of pounds a year on each policy, given the huge disparity between the premium for a 35-40+ year old father and their child who is in their late teens to early 20s. Ignorance of the law isn't a valid excuse, no matter how innocently it may have been done.