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.
The 2020 Year of the Rat bullion coins are part of the Royal Mint's Shēngxiào Collection - a coin series dedicated to the Chinese Lunar calendar.
As the mint says:
"For centuries The Royal Mint has drawn upon its unrivalled heritage, experience, and craftsmanship to evolve and meet the needs of an ever-changing market. In 2014 we struck the United Kingdom’s first Chinese Lunar coins, introducing a new series and a United Kingdom first. The Shēngxiào Collection, named to honour the Chinese zodiac, is a celebration of the UK’s diverse multi-cultural society. At Chinese New Year, gifts and tokens are exchanged, particularly the gift of money in red envelopes, symbolising good wishes for the recipient’s health, wealth and prosperity. These Lunar coins from The Royal Mint embrace this globally observed tradition, lending a unique British angle to an ancient tradition."
"People who are born under the sign of the Rat are thought to be quick-witted, practical, adaptable, kind and intelligent. With strong intuition skills, they always easily adapt to a new environment. With rich imaginations and sharp observation, Rats can take advantage of various opportunities well. Rats are very curious, so they tend to try their hand at anything.
People born under the sign of the Rat have quick minds and dexterous hands, enabling them to make it easier to learn new skills. With an active, almost hectic nature, Rats enjoy a flexible social circle and tend to flit between friendship groups when they please. They are interested in attending different kinds of social occasions and events. They like to make new friends but they rarely have many that are very close. Rats have a charming appearance, which can attract even more friends."
The 1kg silver coins were issued boxed, in Proof FDC grade only (Royal Mint Product Code: UKR20SK, issue price £2050.00, presentation issue limit 28, maximum coin mintage 38). | 
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