This question has fuelled a great deal of debate recently, sadly, with no bullet proof final answer.
To which definition of
'lump sum' do you ascribe; HMRC's or the IRSs'? In your opinion, which definition of lump sum does the treaty assign? Where will you find the definitive IRS statement that pertains to any tax free foreign pension amount? To be certain, are you saying you
did not take a UFPLS option?
Your
verifiable interpretation of the above will determine whether you declare the 25% tax free lump sum amount to be taxable in the US or not. On purpose, I did not take the 25% tax free amount on my UK pension, so avoided the question. My UK company pension contains a yearly COLA enhancement and, unlike yours, is for life.
Is your pension a government pension (I would guess not)? If not, the normal withdrawals from your pension pot (minus any basis) will be taxable in both the US and UK, with tax credits allowed by the US for tax paid to the UK.
For your reading enjoyment:
https://britishexpats.com/forum/usa-57/uk-pensions-related-uk-us-tax-treaty-894492/https://www.taxprotalk.com/forums/viewtopic.php?f=8&t=3780&p=98291&hilit=UK+25%25+tax+free+lump+sum#p98291