I guess I still don't fully grasp how US citizens, resident in the UK, are allowed to re-source US income to the UK on 1116. Technically, does it follow from the worksheet (Additional foreign tax credit on US sourced income) at the back of the 1116 instructions? Does it usually work out that you can simply re-source all of a US-sourced income? From what Sarah says, it seems that this is what people are routinely doing. BTW, when you re-source the income on 1116, do you fill in a separate 1116 and check the "Certain income resourced by treaty" box? Sarah also said in an earlier thread the re-sourcing doesn't have to be disclosed on 8833.
Here is a significant corollary of all this regarding pensions: What if the income was taxable at a lower (or nil rate) in the UK? You would get no foreign tax credit but, because it has been re-sourced, you could pay the US tax on it by using carryovers of unused foreign tax credit that you had accumulated from previous years (< 10). I know for a fact that you can do this if the income really was sourced in the UK (e.g., a lump-sum pension payout that is tax-free in the UK; big accounting firms advise us expats to do just that). But what about, for example, US income tax on a lump-sum pension payout from the US to a UK resident (again, not taxable in the UK). If you re-source it to the UK, you are not paying foreign tax on it so you get no tax credit. However, if it is now UK sourced, you can carryover unused foreign tax credits from previous years and pay the US tax with that. One could argue that, in such a situation, the IRS is, in fact, maintaining its "savings clause" from Article 1; it's just that you are paying your US tax with carryovers of unused foreign tax credits.
Gosh, this stuff can really get technical...I'd be really grateful for comments/criticism from the forum. Could you direct me to any IRS publications that clarify this (as if..)?