If a treaty is included in a treaty series, the treaty series should be cited rather than eg: the UN document number. Treaties are often included in several treaty series. According to Chapter 8.4 of the Australian Guide to Legal Citation, treaties should be cited in the following preferential order:
Consolidated Treaty Series (ConTS) 1648-1919;
The United Kingdom Treaty Series (UKTS) is the UK's official treaty series.
Treaties that have been signed but not yet ratified or acceded to must be laid before both Houses of Parliament as Command Papers, which are published in one of three Foreign and Commonwealth Office (FCO) series:
Once a treaty entered into force for the UK it was published as a Command paper in the UK Treaty Series. In 2020 the United Kingdom of Great Britain and Northern Ireland withdrew from the European Union and the European Atomic Energy Community.
The UK Treaties Online search engine can be used to search all the above series. UK Treaties Online contains the full texts of Command Papers published in the UK Treaty Series from 1892 to current. Full text is only available for UKTS. Records for the other series are available, but not full text.
For detailed information about the treaty-making process and finding the texts of UK treaties, see the FCO's UK Treaties Guidance page
The United Nations Treaty Series Online contains most multilateral and all bilateral treaties in force entered into by the UK since 1949. To find all UK treaties registered with the UN:
For citation purposes, you need to find the UNTS citation (see the box on the left of this page).
The UK follows a dualist approach with respect to the domestic effect of international treaties. This is similar in approach to other Commonwealth countries such as Canada, Australia and New Zealand. The dualist system means that in order for the treaty obligations to be given the force of law domestically, the UK cannot simply become a party to the treaty - the treaty must be incorporated into domestic legislation (Maclaine Watson & Co Ltd v Department of Trade and Trade and Industry  2 AC 418, 474-5).
An example of incorporation of a human rights treaty into UK domestic legislation is the Human Rights Act 1998 c. 42, which gives legal effect in the UK to many fundamental rights and freedoms contained in the European Convention on Human Rights (ECHR).
When the UK is a party to a treaty but has not implemented this domestically by statute, there is a 'prima facie presumption that Parliament does not intend to act in breach of international law', so in cases of ambiguity, the treaty obligations are presumed to prevail (Salomon v Commissioners of Customs and Excise  2 QB 116, 143-4 (Diplock LJ)).
See also Eirik Bjorge, 'Can Unincorporated Treaty Obligations Be Part of English Law?' (April 19 2017) University of Oslo Faculty of Law Research Paper No. 2017-18 (open access).