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, representing the United Kingdom and its constituent parts - England, Wales, Scotland and Northern Ireland, rotating each year; they were all demonetised in 2017 and replaced by the current 12-sided bimetallic one pound coin.
The Royal Mint also uses the denomination to issue Non-Circulating Legal Tender (NCLT) coins for collectors, such as this one.
In 2008, two 14-coin sets were issued to commemorate the 25th Anniversary of the £1 denomination - one in gold, and one in silver with selective gold plating. The sets contained coins dated 2008 with all the previously released one pound designs. There was no base metal version of the set. With the exception of the Royal Arms £1 of 2008 (which was due to be issued in 2008 as the coin representing the whole of the United Kingdom), none of them were released into circulation.
The original design for this coin - representing England - was issued twice, in 1987 and in 1992. | 
Search for United Kingdom: One Pound 2008 Oak Tree and Diadem (NCLT) on eBay 