That is brilliant, and thank you for the comprehensive answer!
Husband and I actually only found ONE tax-advising website in all our searches. Perhaps we read it wrong (and I quote):
'Does the UK tax U.S. social security benefits?
Here’s some reassuring news: under the U.S.-UK tax treaty, U.S. Social Security benefits paid to U.S. citizens and green card holders residing in the UK are taxable only in the United States. This means the UK does not tax your U.S. Social Security benefits, helping you avoid double taxation. However, you must still report your worldwide income—including Social Security—on your U.S. tax return.'
Very misleading, unless, like I said, I'm reading it wrong!
I, like you, am a dual US/UK citizen and reading this shocked me, as I was certain I would need to report it on my UK self-assessment. And on my 1040, which I've done. Everything else I've ever read anywhere else pointed in that direction. And, I've always trusted the advice I receive here.
Thank you!