Hi,
Roth IRAs, these are usually exempt from tax in the USA, under US domestic laws. However, it is the taxpayer’s responsibility to check with officials within the USA to confirm and establish that their Roth IRA is or isn’t exempt from tax in the USA . If the Roth IRA is in fact exempt from US tax, then it will also be exempt from tax within the UK.
This is in accordance with Article 17(1)(b) of the UK/USA Double Taxation Agreement (DTA)
USA: tax treaties
Thank you.