I believe its illegal for the USPS to circulate gambling mail, NS&I is 100% online though. You don't loose your stake, so wouldn't be viewed as gambling, as you have nothing to loose. You will need to file FinCEN114 and note as Securities. Also be mindful of IRS form 8938, as the thresholds fall dramatically when you move Stateside. You will need to look at your specific circumstances, (filling status). and then look at the specified balances to see if you need to file this form. It's a bit onerous, although once you study the process, its straightforward. Keep in mind that all FFI's need to be disclosed, to include pensions and annuities/life insurance polices with a positive balance/ surrender value, so it's your TOTAL foreign assets, (Non U.S.) that will dictate 8938 filing requirements. Unlike FinCEN114, there is no need to disclose POA & signatory authority accounts on 8938. FinCEN114 a Treasury form, more aimed to seek out money laundering. 8938 targets tax evaders (FACTA and the Hire Act)
Report NS&I PB's on Schedule B also and use any 1116 passive FTC's you may have to offset U.S tax exposure. Like ISA's these wrappers and not tax efficient with the IRS.