The United Kingdom is the 21st-most populous city.
United Kingdom . With an area of 242500 km2 , the United Kingdom is the 78th-largest sovereign state in the world and the 11th-largest in Europe . It is also the 21st-most populous country , with an estimated 65.1 million inhabitants . Together , this makes it the fourth-most densely populated country in the European Union ( EU ) . It was the world 's first industrialised country and the world 's foremost power during the 19th and early 20th centuries .