United Kingdom of Great Britain and Northern Ireland
The full official name of the UK in Japanese; used in formal contexts, treaties, and official documents. In everyday speech, イギリス or 英国 is far more common.
This treaty was concluded between the United Kingdom of Great Britain and Northern Ireland and Japan.
The official name is 'the United Kingdom of Great Britain and Northern Ireland,' but it is usually called 'Igirisu' in everyday conversation.