I can't guarantee I've been doing it right, but I've done the same thing, including my and my employer's pension contributions as taxable income, which results in a tax-free cost basis when withdrawing in retirement (you still pay income tax on the growth, but don't have to pay tax again on the contributions). The foreign tax credits due to UK tax on the after-pension pay are still more than enough to wipe out US tax on the pre-pension pay, and probably the UK tax on the 75% taxable part of the pension would eliminate US tax on the pension withdrawals either way - it likely doesn't make much difference.
I didn't differentiate the pension contributions from other Wages, Salaries, Tips, etc. on line 1. I am certainly keeping my own records of what is what, for use in several decades when I withdraw from the pension. I don't see an issue with using Schedule 1 line 8 either, it'll have the same result in the calculations. I did this with TurboTax the last few years - I'm trying TaxAct and OLT this year, but haven't gotten far enough to know for sure if they'll let you do it, although I haven't yet seen any reason to think they won't.