Police have the power to seize a vehicle which is being driven on a public road without insurance . The vehicle is impounded and the driver has 14 days to produce a valid insurance certificate as well as pay the vehicle storage charges. After 14 days, the vehicle may be sold at auction and the owner can claim the money from the sale. In some cases, the vehicle is destroyed , if there is no sale. If your father continues to drive without insurance then you can ring the police and tell them the situation, hoping there may be some action on their part.
Sorry, you can't wait for a driving mistake , or an accident.
To be frank as others have already said, subterfuge is the way forward, if you don't want to go down the police route. If the DVLA says he can't drive then how are you going to deal with this then, if he continues to drive ? Because the relationship will change, it's inevitable. You have to start making often very difficult decisions, with or without POA . I understand you are worried about the reaction of fury , I've been on the end of severe aggression from my mother in law , who had mixed dementia. I think the idea from
@Cat27 of taking the car to your house is a good one and when he reports it stolen, then just explain the situation. I think you will find that the police will see you have acted in good faith, with no criminal intent and see the situation as a family dispute , rather than a dishonest action. Something has got to give here.