I have a Vanguard account from when I lived in the US, but as a mitigation in case Vanguard decides to close accounts of non-US-resident customers, I recently opened a Roth IRA at Interactive Brokers. This was while living in the UK, using only UK information (address, phone, etc.), plus US Social Security number.
No issues at all opening the account, and it feels good to have a backup.
Being able to actually invest in the account is a bit more of a challenge. Because IBKR knows I'm UK resident, they won't let me buy US funds (due to the EU/UK MiFiD rules and lack of a KID). However, I can buy UCITS funds in the IRA. Normally this would be very bad (PFIC!), but since it's in the IRA wrapper, my understanding is that the IRS a) can't impose the punitive tax and b) doesn't need the onerous filings (see
https://www.bogleheads.org/wiki/Passive_foreign_investment_company#US_pensions).
The UCITS ETF does mean that expenses are slightly higher. You should also be aware that you'll be indirectly paying tax on US-source dividends that you don't have to (the ETF pays the taxes, you have no way to reclaim them). If it's an Irish ETF, that's "only" 15%, but if you have the option, you'd keep your US investments somewhere where you don't get stuck with that tax, like a 401k - you get the same problem with UK pensions, etc. Obviously want your overall asset allocation to make sense, but each individual account doesn't have to match the overall allocation.
As durhamlad said, you do need earned income to report on your US tax return. If you have UK earned income and use the Foreign Tax Credit, that does it. Or if you're fortunate enough to exceed the FEIE, the excess would work (but you would pay US tax on it first - at that point, the FTC very likely makes more sense anyway).
And it's not a tax reporting nightmare at all, because the UK recognizes Roth IRA as a pension. Nothing to report at all, although some people make a comment in the free text field on their Self Assessment just to ensure HMRC is aware but there's no tax due. On the US side, it's just like anybody resident in the US, no headache there.