guya: No, I don't have a badge number. That was very quickly spoken at the start. I remember only his first name. Next time I will make a note. Or maybe I'll phone back. But I doubt if he could have quoted from the regulations. You do not say whether or not you think his advice was correct.
My hunch is that he must surely be wrong. When I use this worksheet it computes the fraction of my income that is US source, and then uses that to figure the fraction of my US tax bill that is US source related. But that is crazy, since my US income is dividends, which are taxed at just 15%. The worksheet therefore over-calculates the proportion of my tax bill that is US source related, and then ends up offering me too large a tax credit. The calculation that makes more sense is the one in that is made in finding the adjusted AGI for Line 18 of Form 1116 in which the contribution of qualified dividends to the AGI (net of deductions) is adjusted downwards.
None of my UK source income is taxed by the US (since I have far away more tax credits than I need in respect of UK tax paid.) The only part of my income that can be taxed by the US is US source
qualified dividend income. If that were taxed at 15% I think I would need to pay a smaller amount than it would cost to ask a UK-US specialist to compute it. Maybe I should pay less than 15% to the US. But in any case I am going to pay the balance up 32.5% to the UK, minus the UK dividend tax credit, so my net of tax will be the same - it is really just a question about which country gets what size of share. My preference is to give the largest possible share to the UK, but perhaps it is safest just give the US 15% and then claim that as a credit on my UK self-assessment. I understand that under treaty the US can take up to 15%, although there must be cases in which the US would want less than 15%, as when a person has a low total income.