IHTM11178 | Exclusions from exemption: interest reserved or created - property other than land
From HM Revenue & Customs · Inheritance Tax Manual
Under IHTA84/S23 (4)(b) a gift to a charity (IHTM11112) is not exempt if
the property given is not land or a building, and
it is given subject to an interest reserved or created by the donor other than
an interest created for full monetary consideration, or
an interest which does not substantially affect the enjoyment of the gifted property by the donee.
Whether property is given subject to an interest must be established twelve months after the transfer.
You should discuss the case with Technical before pursuing any tax.