Un-United Kingdom

[Chas]

Depends how far back you want to go.

Geoffrey of Monmouth in his pseudohistorical Historia Regum Britanniae (c. 1136) refers to the island of Great Britain as Britannia major ("Greater Britain"), to distinguish it from Britannia minor ("Lesser Britain"), the continental region which approximates to modern Brittany, which had been settled in the fifth and sixth centuries by Celtic immigrants from the British Isles.[25]

Hardly the same thing.

The term Great Britain was first used officially in 1474, in the instrument drawing up the proposal for a marriage between Cecily the daughter of Edward IV of England, and James the son of James III of Scotland, which described it as "this Nobill Isle, callit Gret Britanee."

Right, the term was used in the context of uniting the crowns of England and Scotland. The point being that this context is not the name "United Kingdom".