At the time of decimalisation in 1971, the United Kingdom Crown coin (equal to 5 shillings) was redenominated as a 25p coin and pieces struck prior to 1990 - all the way back to 1818 - continue to be legal tender for that amount. The old denomination had been a regular circulating coin in the 19th century, but in the 20th century it became more of a commemorative denomination.
The Royal Mint continued issuing commemorative coins in the same format - using the traditional coin dimensions, but in copper-nickel. These were initially also denominated as 25 pence but the "Crown" was changed in 1990 to a commemorative £5 (five pound) coin, having the same dimensions and weight but with a face value twenty times as great.
The denomination is used to mark special occasions, usually royal in theme, rather than for use in general circulation. Although not circulating, these coins are legal tender.
The 2021 Year of the Ox 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 in the Year of the Ox are known for possessing traits such as diligence, dependability, strength and determination. They have a truthful nature and view family and work as very important. Possessing great patience and a strong desire to advance in life, oxen work hard to achieve their goals. They are not easily influenced by others and try to do things in their own way. They almost always have a detailed plan and enjoy great success as a result."
This £5 commemorative coin is only the second copper-nickel coin in the Shēngxiào Collection. It was released in early September 2020, in advance of the Chinese Year of the Ox (11 February 2021 to 31 January 2022). | 
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