The Royal Mint of the United Kingdom, which makes the country's circulating coinage, is also very active in the bullion coinage space and issues many bullion coins which are legal tender.
Additionally, it also makes bullion which is not legal tender and is targeted at investors and not at coin collectors. Some of these products - like this one - have precious metal content of 1000 g (one kilogram, or 1kg) of silver.
These products are rectangular in shape and are known as "gold bars"; they are cast, as their size is not suitable for regular minting techniques.
The Royal Mint says about this product:
"The 1kg silver cast bar from The Royal Mint contains 1 kilo (1000g) of 999 fine silver. It is available for free insured UK delivery or secure storage in The Vault ®, The Royal Mint’s purpose-built precious metal storage facility. Each silver bar is imprinted with a unique serial number and The Royal Mint’s logo." | 
Search for United Kingdom: Silver Kilo Bar 2021 (Royal Mint) on eBay 