The £1 coin in base metal (as opposed to the gold sovereign, which has a nominal face value of one pound too), nickel-brass was introduced in 1983, as a replacement for the £1 banknote. A variety of designs were issued into circulation between 1983 and 2016; they were all demonetised in 2017 and replaced by the current 12-sided bimetallic one pound coin.
Normally, the Royal Mint does not make its pattern coins available to the public. However, there have been a couple of cases when patterns were released to collectors. Another type of coin are the so-called "trial pieces" which are made before a significant change in the specifications of a denomination, such as the switch from a round nickel-brass coin to a 12-sided bimetallic one. They are marked as "trial", have the dimensions and composition of the future coin but have a different design and are not legal tender. The Royal Mint supplies them to different partners such as vending machine manufacturers so they can test their equipment on the new coins. Some of those trials then make it into the hands of collectors.