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.
This commemorative £5 Crown-sized coin was issued as part of a six-coin set of similar coins commemorating the Centenary of the First World War; the set itself is the second part of a series of sets issued over several years. The coin commemorates Edith Cavell.
Edith Louisa Cavell (1865 - 1915) was a British nurse. She is celebrated for saving the lives of soldiers from both sides without discrimination and in helping some 200 Allied soldiers escape from German-occupied Belgium during the First World War, for which she was arrested. She was accused of treason, found guilty by a court-martial and sentenced to death. Despite international pressure for mercy, she was shot by a German firing squad. Her execution received worldwide condemnation and extensive press coverage.
The coin was issued in Proof FDC, in sterling silver and in a gold, both in sets only and not issued individually. | 
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