Yes the tax
I find this whole thing quite bizarre. Imagine the situation where an IRA far outweighs other assets - which is entirely possible. The estate i.e. the executor - likely a beneficiary - could potentially have to close the IRA in order to have enough funds to pay the IHT. In doing so, they would have to pay 30% of the IRA value in tax to the US (lump sum withdrawal), in order to have the funds they need to pay 40% IHT in the UK. It's nuts - assuming it's true.
Agreed, it's not uncommon for IRAs to be a few million dollars, but the US estate tax threshold of $13M means that there's no danger of Federal estate tax and the beneficiaries can take the money out over a number of years, although the IRS has rules about that. If those IRAs are within the estate of a UK domiciled person then a $1M tax bill would not be outlandish and as you point out any lump sum withdrawal from the IRA to pay that bill would also be taxed as income. Another wrinkle is that if the IRA passes directly to the beneficiaries then the estate/executor will have to claw back the money from the beneficiaries. Buzzacotts has some posts about US/UK estate planning, but nothing specific about UK IHT on US pensions, IRAs, 401ks etc. The main way to protect US assets seems to be Excluded Property Trusts set up while a non-dom, but IRAs are never mentioned. That actually gives me a glimmer of hope as no one seems to worry about it.
The potential for large amounts of income tax to also be due on IRAs underlines the importance of TIRA to ROTH IRA rollovers,