The two principal sources for pre 1892 treaties in the UK are:
Consolidated Treaty Series (CTS) covers 1648-1918. (sometimes referred to as Parry's)
British and Foreign State Papers
The Law Bod has the volumes on open shelf on Level Three.
But the Law Bod has collections and tools in other languages, such as
Martens, Georg Friedrich von. Nouveau recueil de traités. 16 vols. Göttingen, Germany: Dieterich, 1808–1842. at Internat 8a. Some of these volumes have been digitized by google, and therefore are available on the free web. Please see box in right hand column
Free online resources to help find treaties to which England/UK was or is a party
The League of Nation Treaty Series is available both via a subscription database and on the free web. See links below:
On open shelf on Level 3, the Law Library has collections of treaties published in and for other jurisdictions.
A key multi-jurisdictional collection commonly called Recueil Martens (after its originator Professor Georg Friedrich von Martens (1756–1821)) is on Level 2 at Internat 8a-8i The google digital books project scanned many of our volumes, which are made available free online from the SOLO lnks below. There are gaps in the coverage - where possible other free online sources have been identified. If you can help fill any remaining gaps please contact us!
Google has also digitised the Law Bod's copy of the 3rd edn Précis du droit des gens moderne de l’ Europe fondé sur les traités et l’usage written by von Marens. (see final link below)
Nouveau Recueil (continuing von Martens, was edited in turn by F. Murhard, C. Murhard, J. Pinhas, C. Samwer, J. Hopf)