You do not have to report anything on your tax return until you make a withdrawal from the UK pension plan.
However, if the value of your UK pension plan exceeded $10,000 at any time during the calendar year you are required to file a FinCen 114 - Report of Foreign Bank and Financial Accounts (FBAR)
FinCen114 is NOT filed with your tax return. For information, please see this TurboTax Help article: How do I file an FBAR report (FinCEN 114)?
For additional information on the US tax treatment of UK pensions, please see this article: UK Pension Plans & US Tax Treatment
