Hi,
Roth individual retirement accounts payments to a UK resident that are not taxable in the United States are not taxable in the United Kingdom. You can see note 2 in the guidance below in relation to other pensions:
DT19852 - Double Taxation Relief Manual: Guidance by country: United States of America: Treaty summary
UK/US citizens resident in the UK are taxable on their IRA interest in the UK. IRAs are treated differently from Roth IRAs, in that they are taxable in the UK under foreign interest. The gross interest would be declared in the Self Assessment tax return, using the supplementary page SA106. The interest would be treated in the same way as UK interest and attract that starting rate of £5000. You can see guidance here:
Tax on savings interest
The guidance at DT19852, in the first link, implies that tax is not payable in the USA on this interest.
Thank you