When you leave the UK, you can continue paying £2,880 into a pension for 5 years and you'll get £720 from HMRC. The question is whether this contribution to a UK SIPP decreases your taxable income in the US ("AGI").
Article 18 of the UK-US treaty seems pretty clear:
Then there's the question whether you have to file Form 8833 to make use of the treaty benefit. I don't know the general answer but in this case (someone on a J-1 as a researcher/teacher), the answer would appear to be no. https://www.irs.gov/individuals/international-taxpayers/claiming-tax-treaty-benefits says: "You do not have to file Form 8833 for any of the following situations: [...] 2) You claim a treaty exemption that reduces or modifies the taxation of income from dependent personal services".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;
All of this suggests you can put the SIPP contribution as a negative amount on line 21 of the 1040.
But when I tried to research this online, it seems there's a lot of confusion and disagreement on how to handle contributions to a UK pension scheme (most of what I found was from US persons living in the UK though whereas here we have a nonresident alien in the US paying into a UK scheme).
