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, then also two kilos (they are actually fractionally heavier as a whole, to account for the small amount of other metal in the alloy).
The 2 kg silver coins are denominated as One Thousand Pounds (£1,000), 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, such as this one.
The Mint says about it:
"The female warrior has been a constant throughout the ages; however, one thing that hasn’t remained the same is the manner in which Britannia coins are struck. In fact, coin production has changed dramatically, from the primitive days of literally striking handheld dies with a hammer to the cutting-edge technology used today.
The Britannia 2021 collection is based around the theme of the Spirit of Innovation and the design itself fully reflects that. Talented illustrator P. J. Lynch took this concept to heart, using the numismatic canvas to innovate how we see the image of Britannia." | 
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