I don't think any conclusions can be drawn about NS&I post-FATCA practice from my experience, given that I'm a pre-existing accountholder. My application probably went through automatically as soon as the deposit cleared. A W8-BEN would have been more informative.
I did notice there's a paragraph in the t&c telling me my data may be sent outside the EEA (but only in conformance with UK Data Protection Law). I don't know if it was there before. Also, nationality is now listed among the items for which a change should be notified to NS&I, so I will do that, and see what documentation they ask for.
Obviously, you will complete the application with your current status: UK citizen only and not a US person.
The EEA requirement has been in place due to the EU Savings Directive, but applicants from outside the EU (but not the US) may be allowed (I don't know).
It will be interesting to see if your National Insurance number has a clinging caveat denoting US personhood within the NS&I records. That receipt for $2,350 from Amsterdam is your most valuable asset until a CLN is issued.
Of course, the US Dept. of State has claimed in the past that you are still a US person until the CLN is issued, although that may be for immigration purposes in the US only. Who knows?