Alderney is one of the Channel Islands (situated in the Channel between England and France), and is part of the Bailiwick of Guernsey - a British Crown dependency. As such, it is not a sovereign country, but has operated as a quasi-sovereign entity since the Middle Ages. The island has issued commemorative coins since 1989, denominated in the Alderney Pound - which is a variant of the Pound Sterling and is legal tender on the islands, but not in the United Kingdom.
The British Royal Mint occasionally uses the jurisdiction of Alderney to issue commemorative coins, usually crowns-sized (i.e. modelled on the pre-decimal crown denomination, which is now continued by the commemorative £5 type). Coins of this type are not intended for circulation.