IHTM34177 | Calculating the loss: capital payments
From HM Revenue & Customs · Inheritance Tax Manual
Capital payments are included in the sale price if
they then sell the investments within 12 months, IHTA84/S181.
The term capital payment does not include the sale price of the qualifying investments but does include any money, or money’s worth, that is not treated as income for income tax purposes.
Sums received from the disposal of rights to a provisional allotment of shares or debentures are treated as a capital payment, IHTA84/S181 (2).