Hi, A lump-sum payment derived by a resident of the UK from a pension scheme established in the USA, shall be taxable only in the USA. Please have a look at the guidance at:
DT19850PP - Double Taxation Relief Manual: Guidance by country: United States of America: Contents
which includes the double taxation treaty. A 401(k) plan is a company-sponsored retirement account to which employees can contribute income, while employers may match contributions. There are two basic types of 401(k)s—traditional and Roth—which differ primarily in how they're taxed. With a traditional 401(k), employee contributions are pre-tax, meaning they reduce taxable income, but withdrawals are taxed. Employee contributions to Roth 401(k)s are made with after-tax income: There's no tax deduction in the contribution year, but withdrawals are tax-free. Employer contributions can be made to both traditional and Roth 401(k) plans.
Thank you.