retirement planning US citizen in UK

I know this has been asked before, but I can't seem to find it. What options does a US citizen working in the UK have for setting back money for retirement?  There would be options with the employer, possibly, but don't those cause trouble with the US IRS?

As the Daughter hopes to be employed someday, and is getting a later career start, it would be prudent for her to sock as much back as she can for retirement while she is living at home. She still maintains mail, etc., at a USA address. I wonder if she could open some sort of IRA in the States, like a Roth?

If she finds an entity that would allow that, how would that impact her UK taxes? I ~think~ I read that if she puts money in a Roth IRA, the UK treats it like any other IRA and does not tax it until it's withdrawn. (?)

On her US taxes, I believe that if she contributes to an IRA with money she earns abroad, she will have to exclude any money that goes into the IRA from any foreign income exclusion she might claim, is that correct?

The Daughter is hoping to apply for citizenship when she is eligible. I would assume that would change the tax landscape for her?

I don’t believe she can open an IRA or Roth and contribute to it based on her UK salary. I may well be wrong of course, and I doubt any US broker would allow an overseas resident to open such an account, but may be wrong.

Looking at IRS publication 590-A it says things like acceptable earned wages as reported on a W2 is valid, and any foreign earned income that you exclude from income cannot be used.

https://www.irs.gov/publications/p590a#en_US_2017_publink1000230352

I believe regular UK employee pension plans are okay to have, as are SIPP’s. I think that if the employer makes tax free contributions to an employee’s plan then those contributions have to be included as income on your IRS return.

I am very interested in the correct answers because my son is here working now and will be completing his first IRS return soon.

Do post if you have more info. We're a few years out from needing it, but it never hurts to prepare in advance.

This link provides the info I think that covers the basics, and the 2 main things I glean from it are as follows for a USC residing and working in the UK and wanting to contribute to a pension plan.

1. For a workplace pension plan if the employee puts in more than the employer it becomes a “foreign grantor trust” which closely resembles a PFIC, otherwise it is treated like a regular pension plan and probably only needs to be reported on an FBAR or 8938 once the thresholds are met.

2. All contributions to the workplace pension plan are immediately taxable by the IRS so both employer and employee contributions have to be reported on the annual IRS tax return.

https://www.goldinglawyers.com/u-s-taxation-on-u-k-retirement-pension-irs-offshore-reporting/

I expect that you already know that cash ISA’s are okay to put money into but the interest is taxable by the IRS each year, and stocks and shares ISA’s (using tracker funds etc) are treated as PFIC’s by the IRS so avoid these.