There are two basic types of 401(k)s—traditional and Roth—which differ primarily in how they're taxed.
With a traditional 401(k), employee contributions are pre-tax, meaning they reduce taxable income, but withdrawals are taxed.
Employee contributions to Roth 401(k)s are made with after-tax income: There's no tax deduction in the contribution year, but withdrawals are tax-free.
Roth Individual Retirement Accounts payments to a UK resident that are not taxable in the United States are not taxable in the United Kingdom.
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.
UK/US citizens resident in the UK are taxable on their IRA interest in the UK.
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 that starting rate of £5000.00.
Have a look at Tax on savings interest
DT19852 implies that tax is not payable in the USA on this interest.