IHTM11173 | Exclusions from exemption: defeasible gifts
From HM Revenue & Customs · Inheritance Tax Manual
Under a gift to a charity () is not exempt if it is defeasible. Broadly, a defeasible gift is one which can be defeated by a future event, such as a gift that could be revoked.
Any gift that:
has not been defeated within the twelve months after the transfer, and
cannot be defeated at any time after this point in time(even if it could have been defeated before the twelve months had passed)
should be treated as not defeasible.