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 gold 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.
This design is the second release in the City Views series and features Rome, the capital of Italy (earlier - of the Papal States and of the Roman Empire).
The Mint says about it:
Our series dedicated to City Views continues its journey around the world with Rome. The coin’s reverse design is based on the engraving "Aspect général de Rome" by Jacques Alphonse Testard, which has been skilfully recreated in miniature. Showing the city as seen from atop the Janiculum Hill in the nineteenth century, it beautifully captures the panoramic sweep of one of the world’s most iconic skylines. | 
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