Hi,
Article 17(2) with reference to Article 1 (4).
If you receive a Lump Sum payment from a pension scheme then this will be taxable only in the country that the payment arises, in this case, the UK.
(
Uk/USA Double Taxation Agreement - 2002).
You would not be able to claim the tax witheld.
Instead you would claim a foreign tax credit in the USA.
Thank you.