Like many other mints, the Royal Mint issues bullion coins in precious metals, having much higher intrinsic value than their face value. Most of these by tradition are measured in troy ounces of fractions thereof, but the mint also introduced metric coins measuring one kilogram of pure metal (they are actually fractionally heavier as a whole, to account for the small amount of other metal in the alloy).
The 1 kg silver coins are denominated as Five Hundred Pounds (£500), although their intrinsic (bullion) value is much higher than their face value. The coins vary in design depending on the occasion (they are "one-year" types usually issued in parallel with smaller denominations with the same reverse designs).
The coins are legal tender but are not intended for circulation. They are targeted at bullion investors or collectors who appreciate the special editions as pieces of art.
Britannia as a symbol - the female personification of Great Britain - 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 “Britannia, the Changing Face of Britain” - a series of one-year design used on proofs made for collectors. In 2022, the Royal Mint also added this Celebration of Womanhood design to the series.
The Mint says about it:
"The exclusive editions of our Britannia coin return in 2022 with a beautiful design by the Italian artist and sculptor Sandra Deiana, who brings together three female figures representing the stages of womanhood.
For centuries, we have been making waves on UK coins, working with the most innovative artists of each age to re-imagine Britannia’s story for every new generation. In 2022, we are proud to release a new, exclusive design created by the talented Italian artist and sculptor Sandra Deiana. This is Sandra’s first collaboration with The Royal Mint and her vibrant, future-facing image portrays not one, but three female figures on the reverse of a Britannia coin for the first time. Skilled in traditional coin-making disciplines, Sandra created the coin relief for her Britannia design in clay. Then, state-of-the-art technology brought her vision to life, resulting in a worthy addition for collectors and investors." | 
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