Devolution in the United Kingdom and its three devolved legislatures

Background

The UK Government's own guidance says devolution "fundamentally changed the constitutional arrangements of the UK" and gave powers and responsibilities to devolved legislatures and governments in Scotland, Wales and Northern Ireland.
Referendums in September 1997 saw a majority of voters in Scotland and Wales choose to establish a Scottish Parliament and a National Assembly for Wales; in Northern Ireland devolution was a key part of the Good Friday Agreement, supported by voters in a referendum in May 1998.
The UK Parliament then passed three devolution Acts: the Scotland Act 1998, the Northern Ireland Act 1998 and the Government of Wales Act 1998.
These established three devolved legislatures — the Scottish Parliament, Senedd Cymru or the Welsh Parliament, and the Northern Ireland Assembly.
Members of those legislatures form the three devolved governments — the Scottish Government, the Welsh Government and the Northern Ireland Executive. Their civil servants do not report to the Prime Minister or to UK Cabinet Secretaries of State.
The UK Parliament retains authority to legislate on any issue, whether devolved or not, but the UK Government has made clear it will not normally legislate with regard to devolved matters except with the agreement of the relevant devolved legislature.
Within the UK Government, Scotland, Wales and Northern Ireland are each represented in the Cabinet by a Secretary of State.
The split of devolved and reserved competence is complex and is not the same in each nation. The UK Supreme Court hears cases on devolution matters under the Scotland Act 1998, the Northern Ireland Act 1998 and the Government of Wales Act 2006.