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Taxation would be different depending whether you request a limp sum distribution OR are taking a periodic distribution.
There is a tax treaty between the US and UK which regulates such situations.
See here - page 19
ARTICLE 17 Pensions, Social Security, Annuities, Alimony, and Child Support
1. a) Pensions and other similar remuneration beneficially owned by a resident of a Contracting State shall be taxable only in that State.
b) Notwithstanding sub-paragraph a) of this paragraph, the amount of any such pension or remuneration paid from a pension scheme established in the other Contracting State that would be exempt from taxation in that other State if the beneficial owner were a resident thereof shall be exempt from taxation in the first-mentioned State.
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.
So - when we are are talking about US pension
-- periodic distributions are NOT taxed in the US and are taxed in UK where you are a resident.
But lump sum distribution is taxed in the US and not taxed in the UK.
Please be aware that you specifically need to claim tax treaty benefits.
If that distribution is taxed in the UK - following rules apply
Taxable pension income Section 575 (as amended by IT(TOI)A 2005) provides that the taxable amount of a foreign pension is 90% of the actual amount arising in the tax year unless the income is charged in accordance with Section 832 of IT(TOI)A (relevant foreign income charged on the remittance basis). As foreign pensions are treated as "relevant foreign income";, Chapters 2, 3 and 4 of Part 8 IT(TOI)A respectively provide for claims to remittance basis, deductions and reliefs and unremittable income. These Chapters contain the rules that formerly applied to Case V of Schedule D.