The United Kingdom is one of 15 rare countries who has Estate or Gift Tax Treaty with the United States.
The full text is here: https://www.legislation.gov.uk/uksi/1979/1454/made/data.pdf
Where property may be taxed in the United States on the death of a
United Kingdom national who was neither domiciled in nor a national of
the United States and a claim is made under this paragraph, the tax
imposed in the United States shall be limited to the amount of tax
which would have been imposed had the decedent become domiciled in the
United States immediately before his death, on the property which
would in that event have been taxable.
This means that residents of United Kingdom are eligible for the 12.06 million USD exemption from US Estate Tax, unlike other non-treaty countries where the exemption is mere 60,000 USD.
For estate amount above 12.06 million USD (prorated US:Worldwide assets), it seems that the US will provide tax credit for "tax paid in the UK" only if the assets were held inside a Trust (i.e. not a SIPP).
When the articles were written in 1970s, the "tax paid in the UK" only meant "UK capital transfer tax". Although Article 2(2) said "This Convention shall also apply to any identical or substantially similar taxes which are imposed by a Contracting State after the date of signature of the Convention in addition to, or in place of, the existing taxes". Thus it should cover UK Inheritance Tax (successor to UK Capital Transfer Tax in 1986). However, SIPPs are not subject to UK Inheritance Tax anyway.
Where the United States imposes tax with respect to property [held in a trust] which is
not taxable in accordance with the said Article 6 or 7 and
sub-paragraph (b) does not apply, the United States shall credit
against the tax calculated according to its law with respect to that
property an amount equal to the tax paid in the United Kingdom with
respect to that property.