HMRC Admin 5 Response

Hi Andrew Glasspool

Payments made by the individual into an IRA, are made after tax relief is given to the individual by the employer.  
Payments from this pension are taxable in the USA.  HMRC do not recognise IRA schemes as pensions, so for UK residents, they are taxed as income under the interest and declared as foreign interest on a tax return (SA106).    
There is no US taxation if the pension is subject and liable to UK tax. If US tax is withheld, then the individual should seek a refund of this tax (file a form 1040NR).  
HMRC will not give a credit for this tax against any UK tax charged on this income.

Thank you