do any of these documents help?
https://www.irs.gov/businesses/the-taxation-of-foreign-pension-and-annuity-distributions
see article 18:
https://www.irs.gov/businesses/international-businesses/united-kingdom-uk-tax-treaty-documents
(1) Subject to the provisions of paragraph (2) of Article 19 (Government Service), any pension in
consideration of past employment and an annuity paid to an individual who is resident of a Contracting
State shall be taxed only in that State
sounds like this means that you are to pay taxes to the US and not the U.K. as you are a resident of the US (a "Contracting State") I don't see anything in that sentence that suggested 'retired', only a pension in consideration of 'past employment'.
does any of this help?
