Your seller is in China, and beyond UK law, but the item was located in Leicestershire where so it might be worth having a word with their local trading standards office. Although they can't prosecute a seller in China trading standards may have the power to seize dangerous or illegal items.
In fairness to the seller, this was a new item and I'm doubtful that the plug would have been factory fitted in this condition. Even in China. I wonder whether it could have been a customer return? If the original buyer had fitted this UK plug It would explain why it wasn't working properly, if that's why they returned it! It's certainly one for the black museum.
