Hi Alan,
Yes, you will have to report your U.K. pension on your U.S. tax return. According to Article 17 of the US-UK tax treaty, the country of residence at the time of distribution has the sole right to tax the distribution, however there is a "saving clause" found in Article 1 (General Scope), under which the United States and the U.K. each reserve a right to tax their own citizens. Due to the saving clause, a U.S. citizen who is a resident of the U.K. and receives a pension payment will be subject to U.S. tax, despite the language in Article 17 of the US-UK tax treaty. https://home.treasury.gov/system/files/131/Treaty-UK-7-24-2001.pdf
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