I see no one has responded to your question which is unfortunate since the relevant topics can be of importance to anyone with a foreign (non-US) pension, whether they use FTCs or FEIE.
The following is my understanding.
If you include the contributions, you can claim FTCs against the tax due and create a basis from which to withdraw from the pension free of US tax in the future.
Claiming both employee and employer contributions to the pension will not necessarily achieve a
fully tax free pension. There is also the yearly growth in the investment to consider. Claiming employee and employer contributions normally achieves a
partial basis.
I'll cover the General Method later, but once the accumulated yearly basis deductions taken equal the amount of the total basis originally calculated, there is no more basis left and yearly basis deductions are no longer allowed (both FTC and FEIE). This usually occurs after roughly 20 years of collecting benefits after retirement (YMMV).
How should this be reported on the US return? Do I report all of my wages + pension contributions on Form 1040 Line 1?
Yes, for the purpose of foreign employment you might treat it as a part of your income (opting out of the treaty). Some might feel comfortable claiming the pension contributions as other additional income on Schedule 1 (adhering to the treaty for 1040, line 1, but then claiming the contributions on Schedule 1). There are often several ways to complete a US tax return and foreign pensions create a very large grey area in the IRS instructions.
Do I fill out Form 1116 with all the UK income tax paid on the total income (wages + pension contributions)?
Yes. The majority of the first page of 1116 are calculations for allocating the standard (or itemised) deduction. With the standard deduction now being $12,000, you want to be sure the ratios representing foreign source income versus worldwide income accurately reflect the stand you are taking. Otherwise, you could get screwed on later calculations which use the results of the standard deduction allocation to determine the amount of FTCs you are allowed.
Do I have to file a statement or something explaining how much of my income consisted of pension contributions?
No, it is not required to file a yearly report (you may if you wish!),
BUT it is imperative that you keep an accurate record for the amounts claimed each year of the employee and employer amounts submitted in your return. When it comes time to claim the benefits of the pension, you will need these. Without them, you will have no basis in the pension. Also, if an audit were to occur, you'll need them!
It is highly unlikely a UK pension will meet the requirements for a US
qualified pension. Nonqualified pensions must use the General Rule (Publication 939). After much consultation of the actuarial tables included and the use of your total contribution figures, the worksheet in 939 will give the amount of basis you use each year once the pension is in the drawdown phase. It sounds complicated but I found it relatively straightforward.
One final note: claiming
only the employee contributions for each year still yields a valid
partial basis and all the above still applies. And for all instances after drawdown begins, you'll need to keep accurate records of the amount of basis claimed each year to calculate the remaining funds available.
https://www.irs.gov/publications/p939