Roth Conversions, changes to taxation based on residency

So I read earlier in the year about the new HMRC guidance re: US pension distributions that appeared to say Roth conversions could now be taxed by HMRC. Any update on what is actually happening in practice?  Also how are they defining ‘lump sum’ now?  I believe there has been an update but I can’t find it.
Finally, with the changes from domicile based taxation to residency based, what, if any, difference does this make for someone like me who has been uk resident >4 years but <10 years?  If there are any moves I can make before rolling over the 10 years I’d like to know what they are.  Also, how is this 10 years calculated? Is it from actual arrival date or is it 10 tax years? Thanks.

None of this applies to me and I don’t know the answers but am interested in seeing the responses.

@Nun may have some good insights

https://talk.uk-yankee.com/index.php?topic=101593.0

https://talk.uk-yankee.com/index.php?topic=101590.0

https://talk.uk-yankee.com/index.php?topic=101605.0

I could not find the official HMRC guidance either but our US based IFA has advised us that even lump sum Roth IRA conversions are now taxable in the UK.
He indicated that the HMRC change was promoted by the IRS making clear that 25% UK tax free lump sum distributions from UK pensions are taxable in the US.
Any thoughts?

I could not find the official HMRC guidance either but our US based IFA has advised us that even lump sum Roth IRA conversions are now taxable in the UK.
He indicated that the HMRC change was promoted by the IRS making clear that 25% UK tax free lump sum distributions from UK pensions are taxable in the US.
Any thoughts?

I can’t point you to an official HMRC directive on this but I believe they now do tax US lump sums including those used to do Roth conversions.

https://www.ustaxfs.com/insights/significant-change-hmrc-stance-on-us-pension-lump-sum-distributions/

On 12th March 2025, HMRC released updated guidance on the UK tax treatment of lump sum distributions from taxable US pension plans, a major shift that has significant implications for US citizens residing in the UK. The US-UK tax treaty has been in place for 22 years, yet only now has HMRC issued guidance clarifying its position on this issue. Previously, it was widely understood that lump sum distributions from US pension plans to a UK resident individual were fully exempt from UK taxation under the treaty. However, HMRC now asserts that these distributions will be subject to UK tax, with a foreign tax credit allowed for any US taxes paid.

What Has Changed?

Article 17(2) of the US-UK tax treaty was interpreted to provide full UK tax exemption for lump sum distributions from US pension schemes.

However, HMRC is now applying the treaty’s saving clause (Article 1(4)) to override this exemption, allowing the UK to tax these payments. The saving clause allows the US and UK to tax their own citizens and residents as if the treaty did not exist, except for certain specific provisions. Given that the saving clause has historically been seen as a US-centric mechanism, HMRC’s new stance could be controversial and may face challenges.

HMRC is also silent on whether the saving clause will be applied in other areas or whether it is specific to Article 17(2).