In the 17th century, the tea consumed in Britain consisted mostly of green tea imported from China. The popularity of tea grew rapidly and eventually matched the demand for sugar as one of Britain's most popular imports. According to P. J. Marshall, who cites Hoh-cheung Mui and Lorna Mui's The Management of Monopoly: A Study of the East India Company's Conduct of its Tea Trade, 1784-1833, around 20 million pounds of tea were being exported from China to Britain in the 1790s. By the 1800s, that number reached 25 million.

The tea trade was very lucrative, but much to Britain's displeasure, the Chinese held a monopoly on tea. The British were restricted to the port of Canton (Guangzhou) in southern China. The Chinese judicial system was considered unreliable and a guild of Chinese tea merchants completely controlled the export of tea. This led to a trade imbalance and a large outflow of silver from Britain to China, hurting the British economy. British officials sought to address this problem. They sent a diplomatic mission led by George Macartney to China, hoping that the Chinese would agree to cede control over a port to the British or reduce the restrictions imposed on British merchants in Canton. Qianlong, the Chinese emperor, refused to offer any concessions. This passage is from a letter from Qianlong to George III:

Yesterday your Ambassador petitioned my Ministers to memorialize me regarding your trade with China, but his proposal is not consistent with our dynastic usage and cannot be entertained. Hitherto, all European nations, including your own country's barbarian merchants, have carried on their trade with our Celestial Empire at Canton. Such has been the procedure for many years, although our Celestial Empire possesses all things in prolific abundance and lacks no product within its own borders. There was therefore no need to import the manufactures of outside barbarians in exchange for our own produce.

But the British did find a commodity that the Chinese desired: Opium grown in India. Additionally, the British discovered that tea could be grown in Assam. In an early example of industrial espionage, the British secretly acquired Chinese tea manufacturing technology through the efforts of Robert Fortune. Fortune convinced Chinese tea manufacturers to emigrate from China to tea plantations owned by the East India Company in Assam. The tea trees brought from China were not suited for Assam, so the Assamese tea plantations switched to native tea trees and hybrids, but the techniques for manufacturing tea originated from the Chinese that had emigrated to India through Fortune.

The British creation of the Assamese tea industry broke the Chinese monopoly on tea. The price of tea dropped and tea became a drink enjoyed by the masses. So it's no surprise that tea from Assam is central to modern British tea culture.

The birth of the Assamese tea industry can even be credited with introducing tea as a recreational drink to India. Native tea plants were originally only seen as a medicinal herb. But when India experienced a surplus of tea in the 1950s, the Tea Board of India began marketing tea as a drink of choice to Indians. The advertising campaign convinced Indians to drink tea by combining tea with milk (the mass drink of North India) and spices.

Sources:

Fan, Fa-ti. British Naturalists in Qing China: Science, Empire, and Cultural Encounter. Cambridge MA: Harvard University Press, 2004.

Marshall, P.J. “Britain without America,” in The Oxford History of the British Empire: Volume II: The Eighteenth Century. Oxford: Oxford University Press, 2001.

Sen, Colleen Taylor. Food Culture in India. London: Greenwood Press, 2004.