hhhhmmm I'm torn on this, in theory i agree with the tax but personally i know of a few cases in which i feel this would be unfair.
My best friend growing up is Polish, her grandparents and great aunts/uncles settled in Chiswick (when it wasn't considered anywhere near as posh as now) as refugees from the second WW. Her grandmother and great aunts managed to get out but sadly the men ended up in camps/very horrible situations. After the war they came over and joined the women living together in large houses, they worked in low paid jobs and bought them.
Now there are a few left alive in their 90s and living in these houses which have lots of the original kitchen and furniture in, as they never could afford new stuff and made do (ie they haven't added any value to it, it is purely the market thru no effort of their own which has increased the price). But the houses which are their homes first and foremost, are valued probably at over £2m.
The inheritance tax would of course be paid on their death (which they are in full agreement), but they don't know how they could afford an extra tax on the house while alive, so would face the trauma of having to leave their homes. I do feel very sad for them in this circumstance and i know it makes them anxious.
Of course this is rare, but in the same way that my grandparents who were in their 4 bedroom council house and were not expected to downsize as it was their home, i think there will be a generation that this will affect unfairly. But i don't know what is the answer.