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. The platinum "piedfort" of this format (same diameter but double the thickness) is roughly equal to three troy ounces of platinum, and is issued quite rarely.
These coins are "Non-Circulating Legal Tender" (NCLT) and not bullion because they are issued at prices much higher than their intrinsic value, and are targeted at collectors who appreciate them for their artistic or sentimental value, and not at bullion investors.