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The US will tax the distribution because you were never taxed on it when the contributions were made and the tax treaty gives the US the authority to tax.Article 17 of the UK/USA DOUBLE TAXATION CONVENTION:"2. Notwithstanding the provisions of paragraph 1 of this Article, a lump-sum payment derived from a pension scheme established in a Contracting State and beneficially owned by a resident of the other Contracting State shall be taxable only in the first-mentioned State."
The UK would not tax you on the lump sum distribution.
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