In the UK the people (IE the electorate) are sovereign. They authorise their representatives (MP’s) to carry out a programme of policies that they approve of (laid out in the MP’s manifesto) by voting for them in an election. Under constitutional law, the Commons holds ultimate sovereignty (on behalf of and authorised by the electorate) and in a unitary state like the UK therefore, the HoC, and not the devolved legislatures, is the supreme sovereign body in the UK. Holyrood, the Welsh Assembly and Stormont (or the London Assembly for that matter) are NOT sovereign parliaments and the UK is a single (or unitary) country, not a ‘union of (sovereign) nations’. It is also important to note that, in a unitary state, as sovereignty ultimately lies with the central political power, such a state is a single country with constituent parts that pool their resources for the common good and not a ‘union of sovereign nations’, because, as previously stated, sovereignty lies solely with the central political authority (the Commons in our case) and remains there, even after powers have been devolved to parts of that country.
In a unitary state, like the UK (IE a country in which there is a single political authority) ultimate sovereignty lies with the central power, in our case the House of Commons. That central authority can devolve any amount of power it chooses to any number of regions of itself, but ultimate sovereignty remains entirely with the central power, which can revoke, in part or full, those powers simply by repealing the Acts of Parliament that set up the devolved powers in the first place. There is absolutely no need under the UK Constitution for the House of Commons to seek or secure any kind of confirmatory vote from the electorate of the devolved region or from the devolved legislature before doing this (though in practice, this would be seen as undemocratic and heavy handed and a confirmatory vote would be sought by the HoC before doing such a thing.) The act of devolving powers to a region of itself in no way confers or transfers sovereignty to that devolved region (as is the case under federalism.)
The principle of the ultimate sovereignty of the House of Commons is confirmed even by the website of the Scottish Parliament, which states:
‘Under this system of devolution Scotland is still part of the United Kingdom and the UK Parliament in Westminster is sovereign (has ultimate power).’
We all live in a country called the United Kingdom of Great Britain and Northern Ireland.