Like many other mints, the Royal Mint issues bullion coins in precious metals, having much higher intrinsic value than their face value. The one-fortieth ounce (1/40 oz) gold coins are denominated as Fifty Pence (50p) and have been issued since 2014.
These tiny coins are legal tender but are not intended for circulation. They are targeted at collectors who appreciate the special editions as pieces of art. The coins are typically "one-year" types usually issued in parallel with other denominations with the same reverse designs.
Britannia as a symbol has been depicted on British coinage for centuries. The bullion series called Britannia has been issued in gold since 1987 and in silver since 1997; it has a "definitive" depiction - used on bullion coins (Brilliant Uncirculated grade), as well as one-year designs like this one - used on proofs made for collectors. In 2018, the artist David Lawrence decided to show her portrait in a classic design, wearing an ancient helmet.
The Royal Mint says:
"A Roman construct, Britannia came to personify this province at the very edge of empire. A female warrior who embodies our sense of self, she encapsulates the characteristics that define us as a nation. Throughout history, Britannia has been a muse for some of Britain’s most talented artists, each one offering their own interpretation of our most recognisable national symbol. For 2018, designer David Lawrence, whose designs include contributions to our five-year series commemorating the centenary of the First World War, restores Britannia to a more classical feel, lending her an added weight of gravitas."
This fortieth-ounce gold coin was only issued in sets, not individually.