An "Aunt Sally". (For those who don't know what an Aunt Sally is, it's a summation which invites others to shoot holes in it.)
The topic was UK stocks and shares ISAs for those with US citizenship.
As nun has repeatedly pointed out, to be free of PFIC constraints, any stocks or shares investment in the UK must have an US CUSIP number. That seems to greatly limit what is available. One also has to navigate the "are you a US Person" question, which may also limit which UK companies allow one to invest. robertUSA has mentioned Charles Schwab which appears to be US friendly. Since there is no mention of ISAs on the Schwab site, one assumes if a stocks and shares ISA was desired, it may require personal contact with Schwab?
We've also not mentioned cost or minimum investments (a consideration which pertains to Schwab and all others).
The gains on the stocks and shares ISA is tax free in the UK, but taxed in the US. There is a £10,000+ tax free threshold in the UK on gains, so the question is: what is the advantage to a UK stocks and shares ISA (if it is indeed possible to have one) for a USC unless the yearly gains are above £10,000+? Is this the point robnw was making?
As for US reporting, someone filing the FEIE (2555) only has the summation of the standard deduction plus the personal exemption as a cushion for the total of all investments (or additional income) if they wish to remain below a threshold in the US with no tax due. For someone filing 1116, it becomes a bit more delicate with a need to have additional taxed investments to help offset any US tax resulting from a UK tax free ISA. This applies to both cash ISAs and a potential stocks and shares ISA.
An additional option is to self-invest in company shares.
Most on this site wish to retain US ties so investing in the US, which now seems a requirement if stocks and shares are involved, may not be an issue. For those who wish to become more UK-centric, it may cause reluctance. There's also the issue of exchange rates. I've lived in the UK through the extremes; exchange rates have varied from $1.05 to $2.05 to the £ during these years.
The summation: is it logical to invest solely in a UK cash ISA for the UK tax free element, and then separately invest in a (US CUSIP) investment knowing the first £10,000/year will be tax free in the UK?