Thanks for this, Guya, and for your efforts to clarify the situation. You, perhaps more than anyone else on this forum, understand the difficulty of the term 'resident' and its implications for a USC. Several of us have made similar inquiries, but always to the lower, front line staff. The one certainty, both from those we communicate with and in the composure of the NS&I written Terms and Conditions, is the lack of definitions of 'resident' and 'US customers'. Not to be critical, but I find the same lack of definition of US customer and resident in the responses from HM Treasury, above. The exact meaning of the word 'resident' is never adequately defined. UK resident? US physically resident? US resident for tax purposes?
According to the Treasury who run NS&I, taxable accounts cannot be held by US persons; which is what the Terms & Conditions say.
This, although somewhat clearer, still leaves a question. Taxable where? Taxable in the UK, or taxable in the US? If the assumption is not taxable in the UK, does this mean a cash ISA is allowable if held by a USC even though it is taxable in the US (ISA rules demand they must be resident in the UK)? Or does it mean taxable in the US? (A guess would be taxable in the UK.)
I've had several requests from banks and building societies for W-9 information. One went to all possible extremes in attempt to answer the question
"why am I considered a US Person". It may have gone too far, since one explanation defined US citizenship. It used the language of the IGA, but didn't consider all possibilities for every individual.
The information sent by one building society was less adequate, and if I were not aware of FATCA (such as an accidental American) and reading their explanations, I would have been tempted to respond with
"this doesn't concern me". It appears not only NS&I, but many of the smaller institutions are struggling. I must confess my dealings with this particular institution was a bit of a trial run. Front line staff, when questioned about my 2nd (American) citizenship responded with
"that's not important, it's just for tax reasons so we'll leave it blank". Six weeks later, I had a request for my TIN, from head office, with the assumption I might have some connection with the US. (I opened an account many years ago with a US passport, my only citizenship at the time.)
For those curious about account balances, all demands for W-8BEN/W-9 have come from institutions where more than $50,000 is deposited. There are others with $50,000+, and so far, I have not heard from any of them. They, also, were opened many years ago with a US passport. (And yes, the IRS is already aware of all of them, via Sch. B, 8938, and FinCEN 114.)