Treatment of US 401(k) for UK IHT

Evening all,

Can anyone provide insight into the treatment for UK inheritance tax of a US-based Fidelity 401(k) held by a US citizen but UK domiciled deceased? I am having great difficulty obtaining any policy documents from Fidelity to understand the level of discretion the Trustees have in making a distribution. I understand there will be US income tax and charges for lump sum cashing out but am struggling to make sense of whether we would treat this as sitting outside the estate for UK IHT purposes as with a (somewhat) equivalent pension investment in the UK, or whether it is taxable.

Any thoughts hugely appreciated.