IRA's are treated differently from Roth IRA's, 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 the starting rate of £5000.00. You can see information here:
Tax on savings interest
DT19852 - Double Taxation Relief Manual: Guidance by country: United States of America: Treaty summary
DT19852 implies that tax is not payable in the USA on this interest.
Roth Individual Retirement Accounts payments to a UK resident that are not taxable in the United States are not taxable in the United Kingdom, see Note 2 in DT19852 above.