If US ETFs [e.g. Vanguard (UK "Reporting Funds")] are not available for a USC-UK Resident in a UK brokerage account, what IS available for a UK-based investment? Only cash and individual shares? Thank you.
The UK broker won't stop you from buying non-US funds, it's just a really bad idea because of PFIC rules (unless inside a "pension" wrapper, including UK pensions and US 401(k), IRA, etc. - not including an ISA). Investment trusts fall in that same bad bucket, even though they look like individual shares.
You can also buy options and other exotic stuff, not that I recommend that!
Summary of the rules and possible paths forward here:
https://www.bogleheads.org/wiki/Investing_from_the_UK_for_US_citizens_and_US_permanent_residents (disclaimer - I wrote most of it, although others have reviewed - not professional advice).