There's been several mentions so far of the taxation of 90% of income, or 10% discount, on foreign pensions paid in the UK.
See page FN9 for the start of the conversation, and page FN10 for a discussion on the 90% rule.
http://www.hmrc.gov.uk/worksheets/sa106-notes.pdfFor example, for US Social Security payments paid by US SS in the UK by deposit into a UK bank account, HMRC will tax as follows:
(Ignore exchange rates for this example)
£10,000 paid in the UK (total according to UK tax year). This is the amount that goes into your UK bank account.
The amount taxable by HMRC will be on £9,000 only. This occurs prior to the application of the personal allowance. If I understand correctly, this is only applicable to those individuals who pay UK tax on the arising basis.
I'll leave all discussion of IRAs and how they're taxed to others.