Explain the situation to the DVLA and then make them an offer for the plate.
You never know you might find someone there who actually has an element of common sense and do you a deal on it.
I would guess legally, as the insurance company paid out on the car, if it had of turned up the car would have been theirs, if your dad didnt include the value of the plate in his claim or make any provision for the return of the plate then that was an unfortunate oversight on his part.
But the fact remains, technically the plate would have been the property of the insurers along with the car.
On the positive side of things they havent just stuck it back in the pot and sold it off at one of their auctions.