As Jayney 12 indicates, the 'registered keeper' of a vehicle is not necessarily the owner. (Many, perhaps most, company cars are registered in the name of the driver even, though their employer, or a leasing company, actually owns the vehicle).
However, unless there's an agreement to the contrary, all gifts are assumed to have been given 'without reservation'. That means that the person making the gift gives up all rights to the item. (Retaining the receipt does not mean that the original purchaser retains any rights over the vehicle. There must have been additional matters for the court to consider in the case which Jayney 12 refers to).
If the purchaser of the car said "I'm giving it to you", it became the property of recipient and the purchaser has forfeited all rights to the vehicle. Only if the purchaser said something like "I'll give you the use of the car" could he reasonably claim to have retained ownership.
Chris