While researching something else, I was reminded of @Swiss_Danny's post re IRAs and IHT. Unless I've misunderstood, in his consultation with Blick Rothenberg he established that an IRA is not included in a person's estate for IHT purposes. Have you read that thread?
Blick Rothenburg confirmed under current rules, IRA are considered pension savings so outside your estate and outside the scope of IHT. However you need to make sure you have nominated beneficiaries. On death they get Legacy IRAs that have to be drawn down over 10 years max and where the drawdown will be taxable in the UK if taken in regular amounts.
From April 2027 this all changes due to Rachel Reeves bringing pension savings into the scope of the estate and IHT. I need to look into to this situation, but this will likely result in the worst of both worlds ie IHT plus the income tax. Which is a blinking outrage