"The Sheriffs Are Coming" type programmes do HPI checks. If a vehicle is found to be on finance they can't take it "because it belongs to the finance company". Sometimes the Sheriffs repossess cars on the behalf of finance companies. I did hear of a case where a guy bought a car in good faith which was under finance and he was allowed to keep it. I can't remember any other details though.
But that is a different situation, they can not repossess property belonging to someone else, but when you buy a car in good faith, the seller is representing that they are the owner of the car and the law seeks to provide some protection to the innocent buyer hence the sale would need to for a valid disposition under HPA '64