As I've said to nun and others many times, I know nothing about 40whatevers and all the rest of the US IRAwhatevers. Never had any as all my pension income (bar US SS) comes from this side of the Atlantic.
That said, there is one thing I suggest you be aware of when talking about worldwide taxable income in the UK (although it may not affect you). Everyone is very pleased about the increases in the Personal Allowance. It's gone up maybe £2,500+ in the last few years. The downside is the 40% tax threshold. That has come down £4,000+ in the same period. If your total taxable income (prior to the PA) is North of £40,000/year, you'll now be very close to the 40% tax band.
Also, once you're over roughly £28,000/year (it's too late to be checking exact figures), there is no higher pensioners PA (which will also disappear completely in the next few years). You get the same allowance as someone working and earning over £10,000/year.
If you file on the remittance basis and leave £2,000+ abroad, you lose the PA completely plus the CG allowance (as I understand it, and I could well be wrong!).
Just something to keep in the back of your mind.