Thank you for your sympathy!
Here is some of the advice that I got from a tax lawyer when I was last in the US in 2019:
“Based on our knowledge of the law and our review of the United States - United Kingdom Estate and Gift Tax Treaty, hereinafter the "Treaty," we informed you that each country has the authority to tax you on your worldwide assets. In order to prevent against double taxation of the same assets by different countries, the Treaty gives primary taxing rights to the individual's country of domicile. The Treaty requires the country of residence to provide a credit for any taxes paid to the country of domicile.
Currently, the federal estate tax exemption in the United States is $11.4 million per individual. Based on the size of your estate and the current status of the law, you will not be subject to tax liability in the United States after you pass away. Furthermore, because all of your assets are located abroad, we do not need to draft any legal documents for the disposition of your estate in the United States.”
I did not, however, ask anything about the sale of my principal residence or holiday home or gifting to my UKC spouse! So these are the outstanding issues.