The official text of any treaty to which the UK is a signatory is published as a command paper. Command papers are a series of parliamentary papers and include other papers such as white papers as well. Command papers in the Bodleian Law Library are in the Parliamentary papers section on the ground floor and are arranged numerically and so you need to have the paper number.
Treaties are also included on the Foreign and Commonwealth Office UK Treaties Online Database and this is a very good place to start your search. UKTO gives access to information on over 14,000 treaties to which the UK is or has been a party with links to texts of command papers published in the UK Treaty Series from 1892. A guide to searching the database is available below the search box. There are also quick links to collections of tax treaties, bilateral and multilateral treaties to browse.
As the UK is a member of the UN and the Council of Europe you may be able to get the text of treaties directly from these organisations. If you are having problems finding a treaty on the Foreign and Commonwealth Office databases it is worth going to the organisation that produced the treaty.