UK/UK Double Tax Treaty - For pensions, pensions income and similar remuneration it is taxed in the country that you live in. In short a UK pension (except government pensions) is taxable in US and not UK if you live in US.
Lump sums are taxed in the same was as they would be in the country where the pension is based. Because 25% of pension fund lump sums are usually tax free in UK, they are therefore not taxable in US.
Foreign Account Tax Compliance Act (FATCA) – under there is a UK/US intergovernmental agreement with most approved UK pensions and investment accounts being exempt from reporting under FATCA in the US (if you are liable to US tax filing).
Warning – be careful if you have a UK Self Invested Personal Pension that you do not invest your SIPP in something that then falls foul of FATCA and you lose the reporting exclusion.
If in doubt, report it anyway.