HMRC IHT treatment of US 401k and IRA assets

Can anyone shed any light on how HMRC treats US 401k and IRA assets that are part of an estate? I’m trying to do some estate tax planning and can’t find anything clear on the subject. For example, if I have £600K in 401k and IRA assets does the estate need to pay tax on the amount above the £325K individual inheritance tax threshold? Or do these "pension" assets pass to the beneficiary outside of the IHT calculations?

Some background - I’m an unmarried dual citizen resident in the UK. Beneficiary is solely a UK citizen.

For sure, UK DC pension plans are currently exempt from IHT.   I sent a question to the Which? support team and got the following answer on UK pension pots including a link to an article of theirs on the subject:

Thank you for your patience in awaiting a response.

I’ve heard from the research team and they’ve advised the following:

"It's true that any money left in a DC pension pot can go to beneficiaries without becoming part of the estate or subject to inheritance tax, but there might still be some tax to pay if the pension holder is 75 or over when they die.

The attached mag article explains this in more detail (p25), and I've also found a piece from our online news story archive that covers the rules in some depth too: https://www.which.co.uk/news/2018/03/do-you-know-who-will-inherit-your-pension-pot/ "
If there's anything else I can assist with, please reply to this email and I’ll be happy to help. Alternatively, you can call our Member Services team on 029 2267 0000, they’re available from 8:30am - 6pm, Monday – Friday and 9am – 1pm on Saturdays.

Not sure about their US equivalents.  I would hope so but don't know for sure.