I am a UK ordinary citizen and resident but also have a "Traditional IRA" pension account in the USA from 1990s when I used to work there.
Last year I have converted a (lump sum) part of this American IRA account into a Roth IRA and paid US tax on this conversion. None of the funds have left the IRA wrappers, this was a conversion from one USA pension vehicle (Traditional IRA) to another (Roth IRA).
Under US-UK tax treaty (article 17 paragraph 2) such transaction is not subject to UK tax.
The question is whether I am required to report it on my UK self-assessment and if so, how.