The half ounce silver piece is a bullion and commemorative denomination issued by the Royal Mint. It is used mostly for Britannia-type coins: British bullion coins issued in gold since 1987 and in silver since 1997. Some of the one-year types are not Britannia-themed though.
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 half ounce silver proof coin was issued in sets only, not individually. | 
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