IHTM12195 | Succession: Simultaneous deaths (commorientes): IHT position
From HM Revenue & Customs · Inheritance Tax Manual
IHTA84/S4(2) applies for the purpose of charging Inheritance Tax where:
two or more persons die in circumstances in which the order of their deaths is uncertain, and
under Law & Property Act/S184 or, prior to 1 November 2016, Succession Act 1964/ S31(1)(b), for the purposes of devolution the younger is presumed to have survived the elder, or
under Succession (Scotland) 2016/Ss 9 & 10, for the purpose of devolution each is to be treated as having failed to survive the other(s).
In this situation and for the sole purpose of applying IHTA84/S4(1), S4(2) overrides the presumption by providing that the deaths are to be treated as simultaneous.
You should note that
S4(2) does not affect the devolution of the estates
there is a similar provision to S4(2) in IHTA84/S54(4) which applies only for the purposes of settled property reverter to settlor relief (IHTM16121)
neither S4(2) nor S54(4) apply to deaths where there is evidence of who died first even if only a very short interval existed between the deaths, and
the effect of S4(2) is considered separately in relation to the deaths of each of the younger and the elder.