There's a whole can of worms here, and we all are aware of the multiple ways accounts, the searches for indicia, and attempts to define reporting accounts may become confusing. A UKC, resident in the US on a green card, may or may not know they are a USP. Judging from posts on the BritishExpats site, many have simply changed their address on a UK account to a parent, sibling, or friends address in the UK. Given that information NS&I, or for that matter any UK FFI, will classify them as resident in the UK and it will be assumed they pay UK tax even though they are in fact a USP. If they have rental property in the UK they may file a UK tax return.
It's the UK FFI's that determine who is a USP, and it's hard to imagine all UK FFI staff being 100% on top of every account and every exact definition as given in the IGA. HMRC only forward the info from the FFIs to the IRS.
I was under the impression that FFIs who are deemed compliant were still required to have a GIIN (FATCA registration number with the IRS) in case they discovered a reportable account. Of the 77,000 (supposedly) FFIs worldwide who have registered so far (as of a week ago), many UK building societies have a GIIN including Chelsea, West Bromwich, Yorkshire, Coventry, Nottingham, and Nationwide. What is surprising is that some, including Derbyshire, Dunfermline, Swansea, and Dudley do not have a GIIN. NS&I does have a GIIN.
I suspect the NS&I misleading (incomplete?) T&C's are only the beginnings of what will prove to be any number of unclear directives and rulings from the UK FFIs. And NS&I are supposedly controlled by the UK government; the same government that signed the IGA.
As for any legal challenges, it will need to be progressed through the European Courts, not a UK Court. See: UK Parliamentary Sovereignty.