Mrtumble - if they haven't sold the house, they haven't made a penny. They don't have £2m, they have something that they could sell for £2m, unless they do, and move away, they don't get the money. You could bring in a tax on the actual money they get, but the fact that my house has increased in £100k hasn't increased my income by one penny, just that if we sold we'd make more, we're not selling so it doesn't make any difference to my life.
It's this attitude that has created a lot of the economic problems we have in my opinion, looking at our homes values and thinking we are rich - houses are unlike any other asset, if I have a painting worth £2m I could sell it and carry on with the rest of my life as it is, no one needs a painting, but everyone needs a place to live. You can't sell, take the money and carry on, you either have to downsize, or move away, distrusting your lives.
We have never taxed potential money, only actual money. It's a dangerous road to go down, not taxing transactions or incomes or profits, but what someone else thinks your profit/income will be at some unspecified point in the future.