@Takumi571 , what @KarenJ2 is referring is to article 18 of the US-UK treaty -- here is the relevant portion of the treaty:
2. Where an individual who is a member or beneficiary of, or participant in, a pension
scheme established in a Contracting State exercises an employment or self-employment in the
other Contracting State:
a) contributions paid by or on behalf of that individual to the pension scheme
during the period that he exercises an employment or self-employment in the other State
shall be deductible (or excludable) in computing his taxable income in that other State;
and
b) any benefits accrued under the pension scheme, or contributions made to the
pension scheme by or on behalf of the individual’s employer, during that period shall
not be treated as part of the employee’s taxable income and any such contributions shall
be allowed as a deduction in computing the business profits of his employer in that other
State.
The reliefs available under this paragraph shall not exceed the reliefs that would be allowed by
the other State to residents of that State for contributions to, or benefits accrued under, a
pension scheme established in that State.
So yes, you can indeed claim your contribution to the pension scheme as an exclusion to your US income ( i.e. you report X-Y as your gross income. But my point was that if all your income in the UK is excludable , you derive no benefit by reducing the income by the contribution -- and you also have the later burden of zero basis in your distributions. That is why I took the more conservative approach of assuming the plan is NOT qualified. You have a choice here
