Thanks, all! This board is really incredible.
I do track all contributions to my UK pension, especially in respect of which contributions I've treated as tax deductible for US purposes and which are not. I understand that the pension will grow tax free, and non-deductible contributions will give rise to a non-taxable distribution in the future (similar to a ROTH IRA) and the deductible portion will be taxable upon distribution (similar to a traditional IRA).
One question about future distributions -- I'm a bit unclear how the US rules treat a "mixed" account. I understand that the UK will tax the entire pension if I am still resident in the UK at that time, so let's assume I have left the UK and have no excess FTCs. As there are two contribution "buckets" in the UK pension, when I receive a distribution in the future, do the US rules treat the fact that the pension has both deductible and non-deductible contributions on (1) a pro rata basis based on initial contribution percentages or (2) a waterfall based on the initial contribution amount (i.e., nontaxable money distributed first, then taxed on remaining).
(2) seems inequitable given that if the account was comprised 100% of nondeductible contributions, the entire pension would be treated as tax free upon distribution.
That being said, I plan to discuss with my adviser whether it makes sense to re-file the previous 2 years to treat all UK pension contributions as non-deductible.