Roth IRA Conversion Query

Be careful assuming that HMRC won't tax IRA to ROTH rollovers that you declare as "lump sums" under the US/UK Tax Treaty. I've read that HMRC is now using the "Savings Clause" to tax the rollover. So talk to a qualified tax professional and maybe calculate the appropriate rollover amount considering both the UK and US tax brackets.

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

I wonder if/when they will start taxing Roth distributions.

Well, the policy seems to have been published by HMRC so any Self Assessment should reflect that and I imagine all tax professionals are aware of the change.