How to report Lump Sum Withdrawal of Roth IRA on Self Assessment?

Hi everyone. Hoping I get get clarification on this. A US/UK citizen resident in the UK Cashed out my ROTH IRA in one lump sum, closed the account and transferred to my UK bank account. My understanding is that this is not taxable in the UK Article 17(2)US_UK Tax Treaty 'Notwithstanding the provisions of paragraph 1 of this Article, a lump-sum payment derived from a pension scheme established in a Contracting State and beneficially owned by a resident of the other Contracting State shall be taxable only in the first-mentioned State.' Do I need to report this on my self assessment and if so where and how? Many thanks

Hi Neil Davey,
Roth Individual Retirement Accounts payments to a UK resident that are not taxable in the United States are not taxable in the United Kingdom. Please have a look at:
DT19852 - Double Taxation Relief Manual: Guidance by country: United States of America: Treaty summary
Note 2 in relation to other pensions.
Thank you.