Thanks for the reply and sorry about the delay in getting back onto this, but here's an update.
I'm a bit confused about the reply indicating that the US tax would be "...bifurcated between contributions & growth." I thought that the IRS assumption would be that any contribution into a (non-Roth) IRA (or previous rolled over 401K) was essentially made from tax deducted income, so income tax would be due on all of the "contributions and growth" when withdrawn? I was hoping that the 25% tax free allowance on pensions under UK law might be applicable via the treaty, but it appears that this was a one time loophole that was filled some time ago, from research elsewhere.
On the definition of a lump sum, I found this post in which is says from a UK perspective, a lump sum cannot be in the same, previous, or next UK tax year as another lump sum: https://www.taxationweb.co.uk/forum/uk-tax-on-us-retirement-ira-t34553.html
However the same post does imply that multiple lump sums can be taken in the US, in consecutive years, although this is an old post and things may well have changed. I do understand that the common use of the term "lump sum" in the US is to take a full IRA distribution, which would clearly not be tax efficient.
So, whether it's defined as a lump sum or not, a ~$44K distribution each year, taxed only in the US and as income at ~12%, seems a logical way forward and how I plan to proceed, starting this year. It may take about 9 years to get all of the funds out (depending on continued growth and rising thresholds) and I can delay my social security, which will otherwise eat into the US taxable income available at the 12% rate, to around 71-72 (73 max I understand, for minimum distributions).