Echoing durhamlad:
ISAs, including LISAs, are fully taxable by the US. Just a brokerage account as far as the IRS is concerned. And since they're outside of a pension wrapper, PFIC rules apply - this essentially limits these to individual stocks, unless you can get somebody to let you buy US ETFs inside an ISA (possibly Interactive Brokers, if you can convince them you're a professional investor). You'd then need to report interest, dividends, capital gains to the IRS and pay US tax - you won't have any UK tax to take as a credit, so you may owe real actual money to the IRS. To me, it's a very close call if it's worth it to avoid the higher UK taxes (only once you get above the pretty generous dividend and capital gain allowances), with the added reporting complexity and reduced diversification of an ISA. That said, I do invest in mine/my wife's, in a diversified basket of 40 UK stocks. But if you decided it was too much hassle and skipped it, that would also be a reasonable approach. I use Hargreaves Lansdown, there aren't many UK brokers that will touch a US citizen (also Interactive Brokers and maybe AJ Bell, I think that's about it).
IRA is a great option if you're using the Foreign Tax Credit, rather than Foreign Earned Income Exclusion - you need some US taxable earned income, but it's fine if you don't actually have to pay tax on the income due to the FTC, you just have the income excluded. I do this in preference to my ISA, maxing this out in early January is on my calendar. You might have challenges finding a broker that a) will open an account for a US citizen outside the US and b) allows you a reasonable selection of investment options, due to the MiFiD rules. One option is buying European ETFs, which are PFICs but are fine inside the IRA wrapper - I do this at Interactive Brokers.