IHTM26202 | Allocating the burden of tax: the rules
From HM Revenue & Customs · Inheritance Tax Manual
The rules contained in govern the allocation of the burden of tax. They expressly override the terms of any Will or other disposition. You may need to know them if you need to value an interest your deceased has in an estate which was partly exempt.
provides that
none of the tax is to fall on any exempt specific gifts () or on any exempt part of a specific gift, and
none of the tax attributable to the value of property comprised in residue () is to fall on an exempt share of that residue