It's all about that bas...is
..... but I imagine this thread will quickly become a discussion on the definition of "lump sum".
Before we come to that point, IMHO, the issues are separate. The question of the 25% lump sum amount, and the answer, is an independent issue. The only question is the future returns of a US person who has contributed to a UK pension while resident in the UK, and has declared the part of their income used for the contributions (or possibly employers contributions as well),
for US tax purposes on a 1040, as
taxable income for US purposes. Do they therefore have a basis in the pension which will,
if the UK 25% tax free amount is claimed as tax free for US purposes, require additional calculations to pro-rate the tax free 25% portion from the remaining 75% taxable portion in order to come to a correct basis for future declarations of UK pension income on the US return? Assume they use the 'general method' to develop the basis.
In which case, could a better(?) option be to not declare as income (via the treaty?), for US purposes, either their UK contributions, the UK employers contributions, and any growth, which would result in 0% basis in the pension, but still allow them (if the 25% lump sum is allowed) to take that 25% tax free and only declare for US purposes the remaining 75% as taxable income?
It's likely all moot due to the higher tax rates in the UK, or the use of, perhaps for the taxpayer, an over generous FEIE in the contributory stage prior to the pension becoming active. Nonetheless, how to handle future US returns may require some serious considerations.
And, what if the taxpayer already has a large, or full, basis in the pension for US purposes? The 25% tax free helps for UK purposes, but not so much for US purposes since they now have UK income, or a portion, which is twice tax free. But, in the overall picture does that matter? Their main gain is the 25% in the UK.
The joy having to declare to two distinctly independent, and completely different, tax regimes and rules (and the joy of the retired individual with a vacant brain and nothing better to do than come up with these preposterous thoughts).