SolvedEIM40032 | Meaning of "the United Kingdom"
From HM Revenue & Customs · Employment Income Manual
The United Kingdom means England, Wales, Scotland and Northern Ireland. Section 1013 ITA 2007 extends this for tax purposes to include the territorial sea of the United Kingdom. The territorial sea means all waters within twelve nautical miles of the shore of the United Kingdom.
United Kingdom airspace is also part of the United Kingdom. This may be relevant when considering where aircrew are performing the duties of their employment.
The Isle of Man and the Channel Islands are not part of the United Kingdom.
See EIM40208 for information on the designated areas for employees involved in the offshore oil and gas industry.