On April 21, 1983, the British government introduced a new £1 coin as a response to several factors, including the decline in the value of sterling and the spread of coin-operated vending machines. Unlike the withdrawn £1 notes, which were worn ragged within nine months, the extra-thick nickel-brass coins could be expected to last for as long as forty years. As of 2010, the Royal Mint estimates that 2.5 per cent of the pound coins in circulation are fake--that's some 30 million coins. By paying attention to a few aspects of the £1 coins that come your way, you can avoid being duped by a fake.
Read the date stamped on the obverse (the 'head') of the coin and then compare that to the design on the reverse (the 'tail.') These designs representing the constituent parts of the United Kingdom are changed every year. On fakes, the date and the design don't match. Visit the Royal Mint website for a chronology of these designs.
Look at the inscription written around the edge of the coin. Once again, these are changed periodically. On fake one pound coins, there is likely to be a discrepancy between the inscription and the date. You can download a file detailing these inscriptions and the dates when they were issued on the Royal Mint website.
Check that the coin's two faces are in line with each other. Lay the coin heads-up next to a ruler with the design parallel to the ruler's edge, then carefully turn the coin over. Is the design still parallel? If it's askew, then the coin is a fake.
Inspect the condition of the coin, comparing it to the stamped date. If it seems rather bright and shiny for its age, that should also trip alarm bells.
An assemblage of fake one pound coins might not be entirely worthless, as counterfeits have long been of interest to collectors.
If you're in possession of a fake one pound coin, it's illegal to use it in a transaction.