Despite execution of a disclaimer with the consequence that the disclaiming beneficiary takes nothing, under Re Parsons [1942] it was held that the disclaiming beneficiary was competent to dispose of the gift between the date of death of the testator and the date of the disclaimer.
This suggests to me that this would qualify as reviewable disposition.
My understanding of this is no; There is no common law duty to accept a gift. A variation of the gift would be a receipt then a disposition of that gift however a disclaimer means you had anything to dispose of in the first place (so it cannot be a preference payment etc). By the same token I believe (although someone please correct me if I’m wrong!) that this means a disclaimer can never be a deprivation of capital for the purposes of the Care Act.
The case of Robson [2014] EWHC 295 (Ch), [2014] Ch. 470, about the BNP trying to redirect a legacy from a foreign testator to its so-called charity without breaching electoral law might shed some light on this distinction that Alex Stanier mentions. The court held that a legatee’s right to seek due administration of the estate was property which could be assigned, but since the party could not have enforced its right to seek due administration of the estate (as that would breached the law on political donations), it had nothing to give away and the variation was ineffective.