Clients often ask whether the correct term is England, Britain, Great Britain or the UK. All these titles can certainly be confusing. With regard to government documents and citations, there are a set of formal guidelines produced by the government Permanent Committee on Geographical Names (PCGN) if you’d like a little bedtime reading. Actually, it is quite an interesting document, with details of the official languages spoken in the UK, their history and the structure of the administrative areas.
When deciding which term is correct, the first criteria is whether the usage is geographical or political (and I’d include legal in the political context). As a political term, the correct title is the United Kingdom of Great Britain and Northern Ireland. This consists of four two countries: England and Scotland, a Principality – Wales, and one Province – Northern Ireland. It does not include either the Isle of Man or the Channel Islands. The term is often shortened to the UK. In a political context, Great Britain is usually used to refer to just England, Scotland and Wales (excluding Northern Ireland).
If you are writing about a geographical area then the correct term is Britain or Great Britain, which covers the main land mass of England, Scotland and Wales, but not the Northern Ireland or any islands. The term the British Isles refers to the mainland of Britain plus Ireland and all the surrounding islands.
If you’re writing in a legal context remember to check which jurisdiction you are citing from. The UK has in effect three legal systems: England and Wales, Scotland, and Northern Ireland. So, if you are referring to English Law, then this is the law as it applies in England and Wales. If your source applies in two or three jurisdictions then it may be more appropriate to refer to the law of Great Britain (if Northern Ireland is excluded) or UK law if it applies in all three. For example, immigration courts have jurisdiction throughout the UK, whereas employment tribunals cover only England, Wales and Scotland.
As with any citation, I like to exercise consistency, so once you have decided on the correct style stick to it, unless there are contextual reasons not to. If you need any help with formulating your citations, proofreading or referencing styles, then get in touch.