Va en inglés. pero una descripción buena de este tipo de falsifiicaciones (si hay problemas con la lengua a la noche os lo traduzco):
Design Errors on One Pound Coins - Forgeries
Pound coins with apparent errors such as the wrong reverse or edge inscription for the year, and/or poor quality edge inscriptions are forgeries.
Some forgeries are plated metal of poor quality, but others are made of brass. These can sometimes be identified by a slight difference in colour to the normal. The inscriptions may be less clear than normal, but as some of the early designs of genuine coins wear very badly, this is not a sole indicator of being a forgery.
A sure test is to examine the bottom of the incuse edge inscription with a powerful lens. The genuine coins show a clearly defined flat base to the hollows forming the inscription, which is almost certainly absent in the case of a forgery.
It has been estimated that 1% of all pound coins in circulation are forged, although in my experience the ratio is a bit less than that.