I'm a USC, UK resident, receiving Social Security. It's 'exempt' from US tax, so you file an 8333, report it as zero on your 1040. You then report it on your UK return and pay the UK tax on it. As I understand it, you can't take a foreign tax credit for this on your US return because you've not included it as income and hence haven't included it in your US tax liability.
But here's my question the answer to which I should know but I'm confused once again: since the UK tax is higher than the US tax, if you do it this way, don't you end up paying more than if you just declared the SS on your 1040, paid US tax, and took that as a foreign tax credit on your UK return. (Simplicity isn't an issue here - Since I have mandatory witholding on my US pension distributions, I have foreign tax credits on my UK return in any event.)