Article: The independence of the Thirteen Colonies in North America in 1783 after the American War of Independence caused Britain to lose some of its oldest and most populous colonies. British attention soon turned towards Asia, Africa, and the Pacific. After the defeat of France in the Revolutionary and Napoleonic Wars (1792–1815), Britain emerged as the principal naval and imperial power of the 19th century (with London the largest city in the world from about 1830). Unchallenged at sea, British dominance was later described as Pax Britannica ("British Peace"), a period of relative peace in Europe and the world (1815–1914) during which the British Empire became the global hegemon and adopted the role of global policeman. In the early 19th century, the Industrial Revolution began to transform Britain; by the time of the Great Exhibition in 1851 the country was described as the "workshop of the world". The British Empire expanded to include India, large parts of Africa and many other territories throughout the world. Alongside the formal control it exerted over its own colonies, British dominance of much of world trade meant that it effectively controlled the economies of many regions, such as Asia and Latin America. Domestically, political attitudes favoured free trade and laissez-faire policies and a gradual widening of the voting franchise. During this century, the population increased at a dramatic rate, accompanied by rapid urbanisation, causing significant social and economic stresses. To seek new markets and sources of raw materials, the Conservative Party under Disraeli launched a period of imperialist expansion in Egypt, South Africa, and elsewhere. Canada, Australia, and New Zealand became self-governing dominions.

Now answer this question: When was Britain called the "workshop of the world"?
1851