If it's a concocted error, I don't think it's an old one. Error collecting has never been as popular in Britain as in the US - for a long time, error coins in Britain were generally seen as defective, and therefore worth less than an error-free example.
What colour is this thing? Apart from the silvery streak, it looks brassy in the photos; that wouldn't fool anyone.
Maybe someone in America or Canada was simply making some King George VI buttons, using a farthing for their master die.

Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis