What about a US registered pension?
The UK has primary taxing authority over UK residents and if you are taxed on an arising basis as a US/UK dual citizen then the UK is not obligated to give you credit for any US tax paid.
On the UK self assessment form,
"If your overseas income has had foreign tax deducted it may be possible to obtain relief from double taxation. Foreign tax credit relief is normally the best way to obtain such relief, but if you do not want to or cannot claim it, you can deduct the foreign tax when calculating the amount of income and gains chargeable to UK tax. You cannot do both"Certain rules apply.
There are also procedures for
"Dividends from foreign companies."Whilst unfamiliar with this process, one wonders about an eventual 'daisy chain' evolving of credits, tax, UK, tax, credits, US, credits, tax, UK, tax, credits, US, .....