On what line of which IRS tax form do I report income from my UK Old Age Pension?

I am a naturalized U.S. citizen (formerly a British citizen) living in the United States. Because I worked for many years in the UK, I am entitled to a UK Old Age Pension. I understand that this pension income is taxable, but I can't figure out how to report this income to the IRS since I did not receive a 1099-R from the British government.