My condolences on your loss.
The policy your late father had on his car is now null and void. The executor / solicitor needs to inform the insurance company and arrange cancellation and refund. If you don't inform the insurance company, they will not pay out in the event of a claim and will only refund the premium from the date they receive the insurance documentation back. If the car is parked up in a public place it must be insured. If it's on private property, it won't be insured if damaged or stolen, but it won't be a road traffic offence.
It should be possible for the executor to transfer ownership of the car to one of the beneficiaries in lieu of cash or other property to the same value and they can arrange their own insurance.
I did this as executor to my late father's estate by getting a written valuation from the main dealer where it was bought new and transferring the ownership to my sister on foot of her signing a letter with the solicitor and the other beneficiaries.
In theory the car could be temporarily transferred into the name of the executor and the insurance premium paid out of the estate, but it gets messy and will have the net effect of devaluing the car due to an extra registered owner.