H & L will sell to Americans, as long as they are not U.S. persons. According to Wikipedia (so, take it with a pinch of salt, but I'm sure I read it elsewhere), using the 1933 regs quoted by H&L, simply being a U.S. citizen does not qualify you as a U.S. person, IF you're not living in the U.S.
https://en.wikipedia.org/wiki/United_States_person#Securities_market_regulationMy understanding of those regs is to basically regulate dealers of securities (although, I have
absolutely no qualifications in U.S. tax or securities law.
I actually emailed H&L last year as I had previously opened accounts for my kids, and this was their reply:
"I can confirm that if you are a full time resident in the UK for tax purposes, then you are eligible to open and operate a Junior Stocks & Shares ISA. The aforementioned declaration regarding confirming that you are not a US citizen refers to being solely based in the United States. Even though you have dual citizenship in the US and the UK, because you are currently solely resident in the UK you would not be considered a US Person by our definition."
Hope that helps.
Also, as pointed out previously, there are some Vanguard funds that do the HRMC reporting. I made a spreadsheet about a year ago that converted the CISP numbers to ISIN (or whatever is required to find them easily over here), and there are some decent options, but none can be used in an ISA wrapper because they are not UK based. So, not sure if it really helps you. Anyway, the spreadsheet must be on my home computer (I might be at work right now), I'll try to upload it tonight so you can have a look