What did the british do to adjust their trade imbalance with china?

The British illegally brought opium into China for sale, and force China to open trade following the Opium Wars.  
Britain had been trying to gain trade access to China as part of its imperial ambitions.  They found a product they could get Chinese people to buy -- the drug, opium.  The Chinese government vehemently opposed this illegal trading the British were carrying on, and the First Opium War (1839-1842) resulted.  The British won and imposed the Treaty of Nanking on China in 1842, which compelled China to open its doors more widely to foreign trade.  The United States followed up in 1844 with The Treaty of Wangxia, which gave the US access to trade in China.  
From 1850 to 1860, conflict continued between the British and the Chinese in the Second Opium War, again with Britain prevailing and forcing China to open trade rules still further.