Robert H. Green Date and Diane Hay (U.S. competent authority & U.K. competent authority) provided clarity to U.K pensions in 2005, especially private and those held in trust under article 3(1)(o). This added to the double taxation agreement that states that pensions are not taxed until distributions are made. So any gains or losses are tax deferred. If you paid tax on any internal gain currently, then you would in effect pay twice, now and when you receive the distribution. From reading the DTA, it would indicate that employer contributions are not considered part of an employee's compensation and you can deduct, (up to IRS limits) your contributions, just like you can Stateside. There was a time that a treaty based return was to be made, although according to the IRS 8833 is no longer needed. Although the form has not been updated since 1996, the “Claiming Tax Treaty Benefits” section from the IRS web site offers exceptions. Anyway this is my interpretation, ask ten people and you're likely to get ten different answers, such is the animal.
“The term “pension scheme” is defined in Article 3(1)(o) as any plan, scheme, fund, trust or other arrangement established in a Contracting State which is: (i) generally exempt from income taxation in that State; and (ii) operated principally to administer or provide pension or retirement benefits or to earn income for the benefit of one or more such arrangements.”
BTW the FBAR is not a tax form, rather an asset reporting tool for the U.S. Treasury.