It's not jut the IR regs, it's also the US/UK treaty - and the fact that in some cases, it's optional, in some, it's not, and in still others it's not applicable. Pensions are very messy!
The after tax portion is being remitted to the UK. The taxes withheld are not. This further complicates the issue.
Your wife may not have to file in the US (there are special rules about electing to pay US tax as a spouse of a US citizen), which make things even more doolally.
This is an area of specialist advice. I recommend you seek a dual qualified US and UK tax specialist to sort out your returns this year. To keep costs down, in future years you could fill it in yourself (following the planning and template of this year's return) and then have an annual "tax health check" consulting session with your dual-qualified specialist to ensure no new tax laws or new treaties affect your filing strategy.