IHTM44074 - Pre-owned assets: election into Inheritance Tax: how to make an election
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
The election must be made using form IHT500, which is the form prescribed by the Income Tax (Benefits Received by Former Owner of Property) (Election for Inheritance Tax Treatment) Regulations SI2007/3000. It should be sent to:
Inheritance Tax
HM Revenue & Customs
BX9 1HT