Thank you very much Guha.
So just to confirm I understand: even if the plan is held by a US relative, HMRC may regard the UK resident child as a beneficiary of a foreign trust, so that would require the child to complete tax reporting and possibly tax payments on the foreign earnings?
A related follow-up question: Does anyone know roughly how much it costs to pay an accountant in the UK to deal with reporting a 529 foreign trust each year? For example, if it would cost us an additional 1,000 UKP each year to handle 529s (500 quid per kid), and we'd have to do this EVERY year until they finish university, then I can see that it might just make sense to liquidate the accounts, pay US tax penalties and forget about it all.
Thank you!