Referring to the other posts saying that UK state pensions are exempt under Article 17, I believe there has been some confusion among the various respondents and experts with respect to the definition of "state pension".
If the pension was earned in conjunction with being employed by the UK government, then Article 17 may apply.
If by saying "state pension" you mean the payment from HMRC which is essentially equivalent to US Social Security, then Article 17 does not apply and the entire amount is taxable. You can ignore all the posts relating to foreign tax credit that may be related to the UK state pension because that is referring to any tax that may be withheld at source before the payment is made. HMRC does not withhold any tax from state pension payments.
Having said that, I believe that if you made voluntary contribution payments to HMRC to add additional years of qualifying service which increased your state pension, you can take a deduction for that part of the state pension which resulted from your contributions. For example, if you had 10 years of work in the UK and paid voluntary contributions equivalent to 5 additional years, you would report the entire amount received as income and then make a negative entry in Miscellaneous Income equal to 5/15 of the payment. If the amount you deduct for this reason is significant then I suggest you contact a tax expert who is specifically familiar with the interaction between UK and US tax regulations.
Also, although taxable income from US Social Security is normally capped at 85% of the income received, that 85% does not apply to a UK state pension. It is fully taxable.
Finally, assuming you are not resident in the UK, you will not need to pay UK tax on the state pension even if you are required to file a UK tax return. The UK will not tax that pension payment to a non-resident, so you can ignore the posts that add to the confusion by referring to taking a foreign tax deduction or foreign tax credit in conjunction with receipt of a UK state pension.
