In a discussion with a financial adviser we shared the view (before the budget announcement) - although I'm sure we don't agree on a number of other political points - that the mansion tax probably wouldn't happen because of the practicalities of arriving at fair valuations. I also understand that if one doesn't have liquid assets to pay the tax that can be deferred until the property is sold, or the owners die (I've not been able to check that) but that may cause a stagnation in the market of those properties and won't bring money into the treasury in the near future?
We also shared the view that the simplest and probably fairest way to raise taxes was to increase the rates payable rather than tinkering with (on not raising!) the tax bands - but of course they felt needed to make and keep that promise ...