The US and UK have a tax treaty that in part says if the pension is a government pension it is taxed in the same way that the US taxes Social Security. You would need to put the income on the SS benefits worksheet, and then put a negative amount in other income with statement "UK pension offset" or something similar. I believe you need to down load form 8833, and attach as a pdf. Might not need form if pension is under $10,000, but not sure you will have to research.

Maybe @itonewbie will weigh in