Thank you for your helpful reply. I haven't come across this to be honest. I'm a mistake USA citizen, born there to non usa citizens. I have become compliant with my taxes over the last number of years, I got all my stimulus payments after getting compliant (totally unexpected and wasnt a factor in me getting compliant).
My Tax accountant (USA based - with many uk clients) hasn't picked up on this and when asked he said I'm allowed to have the equivalent to what he called a 401k (pension plan i understand) in the UK. Ive not reported on this and unsure how to. I also have a bank account and just recently met them about getting a mortgage and I made sure to let them know i was a US citizen and they said that they already knew and that wasnt an issue.
Your tax accountant is quite right about retirement accounts such as employee pension plans (~401ks) and Self Invested Pension Plans (~IRAs) which the IRS won't tax until withdrawals are made.
However, if you want to save and invest outside of retirement plans then that is where you fall afoul of PFIC rules. Even with an ISA the IRS ignores the tax free wrapper and taxes the interest on cash ISAs, and if funds are held in an ISA they will be subject to PFIC taxation. I like Vanguard and hold after-tax investment funds with Vanguard USA, but Vanguard UK doesn't even allow US citizens as customers.
With company pension plans I have heard that as long as you contribute no more than the employer does (ie simply match his contributions) then no IRS reporting is required otherwise it counts as a Foreign Grantor Trust. However, the value of UK pension holdings have to be reported to the US Treasury each year as part of the FBAR filing if you are a US citizen.
ETA
Once you pass the FATCA threshold then you probably have to include your pension plan values on your IRS return as well but I'm not sure about that. (I don't have any UK pension pots of money)
https://www.irs.gov/businesses/corporations/summary-of-fatca-reporting-for-us-taxpayers Taxpayers living abroad. You must file a Form 8938 if you must file an income tax return and:
You are married filing a joint income tax return and the total value of your specified foreign financial assets is more than $400,000 on the last day of the tax year or more than $600,000 at any time during the year. These thresholds apply even if only one spouse resides abroad. Married individuals who file a joint income tax return for the tax year will file a single Form 8938 that reports all of the specified foreign financial assets in which either spouse has an interest.
You are not a married person filing a joint income tax return and the total value of your specified foreign financial assets is more than $200,000 on the last day of the tax year or more than $300,000 at any time during the year.