Emacity DepressedLondoner How do you know the plate's genuine? The class code matches the vehicle, also the Y plates are not controlled at all and can be kept without needing to get them punched.
When the plates get deregistered you either need to turn in the plates, or if you want to keep them you need to get them punched. Y plates are an exception though and can be kept without being punched. There are some ways around it for normal plates too, but I think you'd still have to pay the fine for "losing" the plates.