I don't think anyone is saying that people who own £2m hoses don't own a lot of wealth. They are just saying there are situations where people who do own £2m houses don't neccesarily have the income to match, and will not neccesarily have the money to be able to afford that tax.
There is no way on earth you can believably argue that someone sitting on £2m of assets can't pay a tax worth pennies on that.
I think if I were in the situation where I was going to be expected to pay that much tax, I'd sell my £2m house and buy something smaller here then buy something else in another country.
Great - that's 2 houses bought, instant stamp duty boost. that's more than they'd squeeze out of you any other way.
Introducing taxes that people perceive to be unfair, whether some people think they are fair or not, just encourages the biggest contributors to become more imaginative when it comes to avoiding paying tax.
The last 20 years or so have been a marvellous time to be very rich in Britain and the US, the % of the wealth owned by the top few % has increased massively while the tax take on them has fallen. A mansion tax or similar falls well short of getting to EU levels of tax on the weakthy. Those that bailed out the financial system are by and large those who did not benefit from it before, nor after.
I think you will find that most people perceive that to be unfair, whether those sitting with the wealth think it fair or not.