Link 1: How to spot a fake £1 coin | Money | guardian.co.uk

...If your change rings alarm bells, you should ask the person who handed it over to give you a different coin, because once it's in your pocket you shouldn't spend it – it is illegal to do so. What you should do, says the Mint's spokesman, is take it to the police, "that way we can find out where the hotspots are". Sadly the police will not give you a genuine £1 coin in return.
Link 2: Britain's £1 coin designs

Link 3: A guide for identifying genuine coins