Two other options come to my mind for a US citizen living in the UK. They could just buy shares in individual companies with a UK broker or buy UK EFTs or mutual funds and deal with the PFIC implications. If they do this inside an ISA there will be no UK tax and then they can do a QEF declaration and just pay the tax each year to the IRS.
I do this for my kids, as their UK grandfather put some money into JISAs for them. To save the crushing weight of paperwork, I've moved them all onto only one tracker, so only one f8621 each year (mark to market delcaration). Also, if there is some new money (another gift from their Grandad for example), I just sell everything in late December and then use the new money combined with the proceeds from the previous year to buy more shares of the fund in the new year. Otherwise, I'd have to do a f8621 for each set of stock purchases.
A few things to note:
* For US citizens, the only full service UK brockerage I can find is H and L. Most, even Vanguard, don't want to know you if you're a US citizen, too much paperwork!
* From what I can tell, if your dependent child has more than $1400 in theoretical earnings from their m2m declaration in form 8621, then they'll need to file (or you need to file on their behalf) a form 1040. In 2021, given some crazy exchange rate swings, two of my kids had to pay tax on their JISA investments.
* Please join ACA and write your Congressperson. The US needs to adopt residency based taxation. If you have lived and worked in both countries, then it might not solve everything as you'll have ongoing retirement account admin and the like in both countries, but I've only every really been a proper adult in the UK (came over here at 24 to study, and met my partner), so my US ties are almost nil, but I am always very conerned about every financial decision because I know it might cause me shedload of paperwork with the IRS, and if I get it wrong, shedloads of fines!