British treaties are published as Command Papers, with both a command number and a treaty series number (e.g. Treaty #1 (1973) has a command number: Cmnd. 5179. Treaties for a given year are listed in the HMSO (Her Mahesty's Stationary Office) annual and monthly catalogues by command number.
Index of British Treaties DBW govt JX 636 1892 Index.The four-volume set covers 1101 - 1988. Lists British treaties for this time by country.
Index to Treaty Series DBW govt JX 636.A3. These volumes index the years: 1968-1982,1985-1989,1991-1992.
British Parliamentary Papers (1731 - current). DBW govt mcard and mfiche UK1 PA B67. Treaties are indexed annually in the British Parliamentary Papers on the last microcard in each box. The text of the treaties is also included.
From 1800 - current, this set is online, House of Commons Parliamentary Papers (see below)