IHTM11303 | Armed forces on active service: meaning of active service
From HM Revenue & Customs · Inheritance Tax Manual
A member of the armed forces is on active service when they are on
active service against an enemy, IHTA84/S154(2)(a) or
such other service of a warlike nature in which in the opinion of the Treasury involved the same risks as service of a warlike nature, IHTA84/S154(2)(b).