Hi,
Traditional 401Ks are taxable.
Roth 401Ks are not.
Article 17(2) of the UK/USA Double Taxation Treaty (
USA: tax treaties) provides the US with the right to tax any Lump Sum payment which is made from a US sourced pension scheme (including IRAs).
However, the UK is also permitted to tax the same lump sum payment(s), which is in accordance with Article 1(4) of the DTA.
You will need to contact the inheritance tax helpline on 0300 123 1072, as this is a different tax to income tax.
Thank you.