The famous case of Richard Burton is worth considering. He lived most of his life in USA and Switzerland. When he died, he wanted to assume Swiss domicile. He was ultimately buried in Switzerland as the domicile he wanted for himself. However, he had a Welsh flag draped over his coffin, and based upon this simple act, HMRC deemed him Welsh and bought his worldwide estate into UK IHT. In short, it's difficult to predict.
As for HMRC agreeing to settle on a smaller sum, the rates of tax are clear. You may hear of HMRC "doing deals" with the likes of Vodafone, but these are usually tax structures built within the law, but exploiting certain things to deliver an advantageous result. A sort of very clever tax mitigation scheme. These are available to individuals with deep pockets, although there are no guarantees that these stuctures will work. We may be getting a General Anti Avoidance Rule, which basically says "if it's clever and delivers an unexpected tax saving, we'll ignore it and tax it as we feel it should be taxed." We currently have a piece of case law that says something similar. The Ramsey principle, says that if a planned series of events combine to make a saving advantageous to the taxpayer, when there is no commercial purpose to the events, HMRC will look through these steps to see what was trying to be achieved. If the key objective is to save tax, HMRC will ignore the steps and tax it as they feel it should be taxed.
However, if your estate is suitably large, trusts are an excellent and simple way for your family to savoid tax relatively simply. You just need to identify how much of your estate you need access to and plan with the rest. Lifetime gifts are extremely worthwhile and efficient. "It's nicer to give with a warm hand than a cold one".