how do I report UK state pension on a 1040?

In TurboTax Online, If the pension income is to be reported, submit a substitute 1099-R as if the payer did not submit a statement when it was required.

• Down the left side of the screen, click on Federal.
• Down the left side of the screen, click on Wages & income.
• Click the down arrow to the right of Retirement Plans Social Security.
• Click to the right of IRA 401K Pension Plan Withdrawals.
• At the screen Let's import your tax info, select Change how I enter the form.
• At the screen How would you like to upload your 1099-R, select Type it in myself.
• At the screen Who gave you a 1099-R, select  Financial institution or other provider.  Click Continue.
• At the screen Enter  your 1099-R providers info, enter the information.
• Enter the dollar amount in box 1 box 2a and select '7' for box 7.
• At the screen Do any of these situations apply to you, select I need to file a substitute 1099-R.

See also this TurboTax Help that discusses the issue.

United Kingdom Tax Treaty, Article 17, page 19, discusses the circumstances when the pension distribution is taxable on the Federal 1040 income tax return.