The image is of the "Order of the Garter" and the lettering on the garter reads "HONI SOIT QUI MAL Y PENSE". The Order of the Garter is an order of chivalry founded by
King Edward III of
England in 1348 and
still awarded by the monarch today. It is the order of knighthood in the British honours system.
That said, I haven't a clue what it is but I very much doubt that it is a coin. The
Gothic lettering below the Knight's helmet might resolve it but my ancient eyes just can't make it out. Turn the image anti-clockwise 90º for the right way up.
Alex