The British pre-decimal penny coin, abbreviated as 1d (from "denarius" - the Roman coin from which the penny is directly descended), was a unit of currency that equalled one two-hundred-and-fortieth of a pound sterling (there were 20 shillings to a pound, and 12 pence to a shilling so one pound was equal to 240 pence).
The denomination continues the tradition of earlier penny coins of Great Britain, which also featured the figure of Britannia. In 1801 the parliaments of Great Britain and the Kingdom of Ireland each passed an Act of Union, uniting the two kingdoms and creating the United Kingdom of Great Britain and Ireland. Re-coinage followed in 1806 and these coins are part of it; they are known as the "Fourth Issue" copper coinage of King George III, the first three coinages being while the country was still just Great Britain. It is also known as the "Second Soho design" (the first being the Cartwheel Penny of 1797).
Between 1770 and the end of the century the practice of counterfeiting had become so prevalent in England that scarcely any genuine British copper coins remained in circulation. The 1770-1775 issue was melted in huge quantities and made into lightweight counterfeit coins; the only other pieces in circulation were the merchants' tokens issued by private firms for their own convenience.
It was during this period that Matthew Boulton offered a solution to the problem by proposing that (1) each coin should contain its intrinsic value of metal, (2) a retaining collar should be used to maintain a constant diameter and (3) a broad raised rim should be used to save the coin from undue wear. He further proposed that a steam powered coinage press be used to produce a more uniformly finished coin with a greater rate of output. A total of £310,885 worth of pennies were coined by Boulton at the Soho Mint in the years 1799, 1806 and 1807; a unique piece dated 1808 is also known. The dies were produced by Conrad Heinrich Küchler, a talented Flemish die cutter. The faces of these coins are slightly concave to protect the design from wear and prevent counterfeiting. The design is identical to that of the half penny and farthing coins minted in the same years; the difference is only in size. Typically for British coinage of the time, the denomination (or, indeed, the country of issue) is not spelled out on the coin.
This format of the denomination was short-lived. After 1807, no penny coins were issued until 1825 (except for another short-lived penny experiment with a circulating silver format), when a smaller version was introduced, with a different design and traditional flat surfaces. All these pennies were minted in copper, but after 1860 the denomination changed to bronze with an even smaller size; all British copper coinage was demonetised after 31 December 1869.