Thanks for posting that link CathyorClaire, as it confirms exactly what I posed earlier:
The inheritance section of the memorandum begins by noting that some royal assets are held by the Queen as “as Sovereign rather than as a private individual” and that “it would clearly be inappropriate for inheritance tax to be paid in respect of such assets”.
Assets of this type include official residences such as Buckingham Palace, the Royal Archives, the Royal Collection of paintings and other works of art.
They are not really the King or Queen’s personal property, but that of the crown and are thus not subject to inheritance tax.
In addition to that (which is the vast bulk of the assets that would be subject to notional tax) what the Queen does hold as personal property is also exempted from inheritance tax.
Two justifications are given for this: first, it says that the nature of the monarch’s role means that it is important for them to have “sufficient private resources” to enable it to continue to perform its traditional role in national life, and to have a degree of financial independence from the Government of the day.
Second, it notes that some of the monarch’s private assets are also used for official functions - private assets such as Sandringham and Balmoral have official as well as private use.