What is the United Kingdom — and how is it different from Britain?

The United Kingdom is the sovereign state. It consists of four nations: England, Scotland, Wales and Northern Ireland. Its full name is the United Kingdom of Great Britain and Northern Ireland, and that name is itself the explanation — Great Britain plus Northern Ireland.

Great Britain is an island. It contains England, Scotland and Wales, and it does not contain Northern Ireland. When a statistic or a law applies to 'Great Britain' rather than 'the UK', it is excluding Northern Ireland — which happens more often than people expect, because Northern Ireland has separate legislation on many matters.

England is one nation within the UK, and by far the largest: about 84 per cent of the population lives there. Using 'England' to mean the UK is the error that most reliably annoys Scottish, Welsh and Northern Irish people, and it is also frequently a factual mistake, because English rules on health, housing and education often do not apply elsewhere.

The British Isles is a geographic archipelago containing Great Britain, the island of Ireland, and thousands of smaller islands. It therefore includes the Republic of Ireland, which is a separate sovereign state and was never part of the UK after 1922. The term is contested in Ireland for exactly that reason; 'Britain and Ireland' is the neutral alternative.

Two further complications. The Isle of Man and the Channel Islands are Crown Dependencies — self-governing possessions of the Crown, not part of the UK, with their own laws and tax systems. And there are fourteen British Overseas Territories, from Gibraltar to the Falklands, which are also not part of the UK.

The asymmetry is the striking feature. England has no devolved parliament of its own — the UK Parliament legislates for England on devolved matters, which is why an 'English' policy is announced by a UK minister. Scotland, Wales and Northern Ireland each have their own legislature with different powers.

Scotland has the most extensive devolution, including its own legal system, its own income tax rates on earned income, and increasing control over social security. Wales has a legislature whose powers have grown substantially since 1999. Northern Ireland's Assembly operates under the power-sharing arrangements of the 1998 Belfast (Good Friday) Agreement and has been suspended for extended periods.

The UK has no single written constitution. What it has instead is an accumulation of Acts of Parliament, court decisions, treaties and — crucially — conventions that are binding in practice but unenforceable in law.

The foundational principle is parliamentary sovereignty: Parliament can make or unmake any law, and no Parliament can bind its successor. There is no constitutional court that can strike down an Act of Parliament as unconstitutional, which is the sharpest difference from the United States, Germany or Australia.

In practice this means constitutional change happens by ordinary legislation. Devolution was created by Acts of Parliament in 1998. The Supreme Court was created by an Act in 2005. The relationship with the European Union was created and then ended by Acts. A written constitution would have made each of those far harder.

The system depends heavily on people behaving as convention expects. That works well until it does not, which is why constitutional argument in Britain tends to be about whether a convention exists and what it requires, rather than about what a document says.

The UK covers 243,610 square kilometres — smaller than most people expect, and roughly comparable to Oregon or Ghana. No point in Britain is more than about 113 kilometres from the sea.

The highest point is Ben Nevis in Scotland at 1,345 metres. England's highest is Scafell Pike at 978 metres, Wales's is Snowdon (Yr Wyddfa) at 1,085 metres, and Northern Ireland's is Slieve Donard at 850 metres. These are hills by international standards; the terrain that makes them serious is the weather.

The population is unevenly distributed even by European standards. England has roughly 434 people per square kilometre, among the densest in Europe, while the Scottish Highlands are among the emptiest inhabited areas in the continent.

The country sits between about 50 and 61 degrees north — the same latitude as Labrador and Moscow — but is kept temperate by the North Atlantic Drift, which is why it rains constantly and rarely freezes hard.

Reserved to the UK Parliament, and therefore the same across all four nations: income tax administration and National Insurance, immigration and nationality, defence, foreign policy, monetary policy, most employment law, most consumer law, company law, and broadcasting.

Devolved, and therefore different: health, education, housing, planning, local government, transport, agriculture, the environment, most of the justice system in Scotland and Northern Ireland, and — increasingly — social security in Scotland.

Some things fall in between in ways that surprise people. Scotland sets its own income tax rates and bands on earned income while HMRC still collects the tax. Prescriptions are free in Scotland, Wales and Northern Ireland but charged in England. University tuition fees differ sharply between the four nations, including for students from other UK nations.

The practical consequence for anyone looking something up: check which nation the source is describing before relying on it. GOV.UK covers the whole UK for reserved matters and England only for many devolved ones, and it does not always say so prominently.