Sorry, that's not quite clear to me as it's not quite in the terms I put the question. So that I can be sure, are you saying that it is just a matter of the executor liquidising enough estate assets to pay IHT, then paying it, following which the estate can be disposed, and that the Insurance policy with my son as beneficiary has no point as the IHT tax planning device I was told it was?
Your son's receipt from the insurance policy is outside the scope of your IT. Let us suppose he receives 10K. You expected your net estate to be 50K and it actually was 40K. The insurance would make up the shortfall. That is a form of IT planning. Your executor will indeed proceed as you suggest. That is his duty!
If the IT is higher than you planned then the insurance policy your son holds will cushion the reduction in the anticipated estate.