Hello and thanks for trusting me to help you today. I am a tax adviser with over 15 years of experience.The US UK tax treaty has been updated foe certain distributions from pensions (IRAs included).
Under the old Agreement, a lump-sum payment from a pension scheme was taxable only in the country of residence. So if an individual moved from the US to the UK before receiving a lump sum from a US pension scheme, they would be taxable on the lump sum neither in the US (because of the treaty) nor in the UK (which does not tax lump sums anyway).
The new provision prevents this occurring by providing that a lump-sum payment derived by a resident of one State from a pension scheme established in the other State shall be taxable only in that other State.
SO you will be taxed in the US on the distribution but not in the UK if it is a lumpsum distribution.
If you are a US citizen you are taxed on the amount anyway (because you are US citizen). The residency in UK would cover you for the lumpsum distribution from taxation in the UK.