I am not sure how Apple will go with the serial number requirement.
The have identified a potentially serious electrical hazard and the recall should not be dependant on you having
a valid serial number to go with it. The hazard remains, serial number or not.
This can easily fall under the consumer Guarantees act "Free from defect" clause.
We have multiple devices and I have kept these plugs over the years eve through the computer they originally came from has since died.
They have been mixed and matched as we have travelled, shifted house etc so I can not say what adaptor came with what device.