Suggested starting point: chapter 29 of Oxford Handbook of the History of International Law
Subject searches to try in SOLO
International law -- China -- History
China - foreign relations
China - history
Great Britain -- Colonies -- Asia -- History
Great Britain -- Territorial expansion -- History
Jacques deLisle 'China's Approach to International Law: A Historical Perspective' (2000) 94 Proceedings of the Annual Meeting (American Society of International Law) 267 at Internat 300 A110 (Holders of an Oxford SSO can access online via link for the journal below.)
First Opium War (1839-1842) ended with the western powers and China signing "unequal" treaties of "Peace, Amity and Commerce"
such as the Nanking Treaty of 1842 between China & GB (93 CTS 465) and that of 1844 between China & US (97 CTS 105)
Second Opium War (1856–1860)
Tientsin Treaties 1858 such as that of "Peace, Friendship and Commerce" between China & GB 119 CTS 163
Research in this topic may well lead you to discover the
Bodleian K B Chen China Centre Library
Dickson Poon Building
Canterbury Road
Oxford OX2 6LU
This is about a mile (20 mins walk) from the Law Bod, to the north (away from the city centre).