What is devolution and what does Westminster still control?
Devolution is the transfer of specified powers from the UK Parliament to the Scottish Parliament, the Welsh Senedd and the Northern Ireland Assembly. It is not federalism, and the distinction matters.
The reserved powers model. For Scotland and, since 2017, Wales, the arrangement works by listing what is reserved to Westminster. Anything not reserved is devolved by default.
Reserved to Westminster:
Defence and the armed forces.
Foreign policy and international relations.
Immigration and nationality.
The constitution, including the union itself.
Most taxation and fiscal policy, monetary policy and currency.
Most social security and pensions.
Trade and economic regulation, competition, consumer protection.
Energy regulation, telecommunications and broadcasting.
Devolved, broadly:
Health and the NHS — which is why NHS structures, prescription charges and policy differ substantially between nations.
Education, including different qualification systems.
Housing, local government, planning and transport.
Justice and policing in Scotland and Northern Ireland; policing is devolved in Wales only partially, and justice is not.
Environment and agriculture.
The asymmetry is the defining feature. Each settlement differs — Scotland has the widest powers including a separate legal system that predates devolution; Northern Ireland's arrangements arise from the Good Friday Agreement and include power-sharing requirements; Wales's powers have expanded incrementally and remain narrower.
England has no devolved parliament, which is the long-standing gap sometimes called the West Lothian question.
The constitutional position, which is genuinely important: because of parliamentary sovereignty, Westminster retains the legal power to legislate on devolved matters, and could in principle amend or abolish the devolved institutions. The Sewel Convention provides that it will not normally legislate on devolved matters without consent — but the Supreme Court has confirmed it is a political convention, not legally enforceable.
Combined authorities and metro mayors are a separate English arrangement, involving delegated rather than devolved powers.