@Barcrest - comments in relation to the 25% tax free lump sum - I do think there's a tendency to overthink some of these things and ultimately live in fear of the thud on the doormat as that letter from the IRS comes through the letterbox! - I've had my own experience of that (see some of my other posts - very unnerving, but the IRS was wrong, I fought it and won and saved myself $30k on an incorrect AMT passement on a UK Capital Gain)
The 25% tax free commencement lump some is an area i've not been able to clarify with regards it being subject to tax by the IRS or not. The simplest option is not to take that 25%, then you're in the clear going forward.
@tubaleiter is correct when he states:
The general consensus seems to be that the 25% tax-free lump sum is US taxable, but there are some dissenting voices out there, so I don't consider it a completely settled question. Assuming that the 25% is US taxable, definitely makes sense not to take it as a lump sum up front, and to use either UFPLS or flexi-access drawdown. Then, 25% of your withdrawals will be UK tax free. At current tax and exchange rates, UK tax on 75% of the payment is typically a bit higher than US tax on 100% of it, so Foreign Tax Credits would probably cover the US tax on that 25%.
I disagree however on the simple maths of taking the full lump sum in one go:
Here's a couple of calculators to help illustrate the point:
UK Calculator
https://www.which.co.uk/money/pensions-and-retirement/pensions-retirement-calculators/pension-tax-calculator-af1sc5u1y9mcUS Calculator
https://www.forbes.com/advisor/income-tax-calculator/As an example (basic, assuming just the lump sum as income) - a UK £500K lump sum - £375k is taxable and results in a UK tax liability of £153,710 - or $184k @£1=$1.20
That same £500k is $600k and that results in a US Federal Tax liability of (assumed married) of $150k - so $34k that can then be used to carry forward in your carryover worksheets.
Then of course, there's any accumulated and un-utilised foreign taxes brought forward form the previous 10 years. I can't currently think of any circumstances where a US tax liability could arise on UK lump sum, other than a dramatic strengthening of the £ rate against the dollar. Using the calculators noted above, breakeven occurs at about £1=$1.40
My biggest concern would be avoiding the UK income tax on the 75% portion that is taxable, and here I agree with
@tubaleiter that flexi-access drawdown is the best method - it really depends on your own 'pain threshold' of how much UK income tax you're prepared to pay.