I was wondering why the UK was not called a "federation".

After some back-and-forth, ChatGPT clarified:

In a federation, there's typically a constitutional division of powers between the central government and the subnational entities (states, provinces, etc.), and this division is usually fixed and applies uniformly to all members of the federation. Each level of government has its own sphere of authority, as defined by the constitution, and these powers are often enshrined and protected by law.

In contrast, the UK's system of devolution is more flexible and asymmetric. The powers devolved to Scotland, Wales, and Northern Ireland have been granted through separate pieces of legislation (Scotland Act, Government of Wales Act, Northern Ireland Act), and the specific powers vary between them. Additionally, these powers can be amended or repealed by the UK Parliament if there is sufficient political will to do so. So, while devolution grants a degree of autonomy to subnational entities, it does not involve a fixed constitutional division of powers like in a federation.