Why did Britain demand a change to its trade agreements with China during the age of imperialism?

Because Britain wanted to expand its manufactured goods market. In the eighteenth century, Britain traded English wool and Indian cotton for Chinese tea and textiles; however, when the Chinese demand weakened, Britain demanded other means of attracting trade with China. Britain realized it could make up the trade deficit with China by selling Indian opium into the Chinese market, making opium the most profitable and popular crop in world markets.