Location, location, location
It's not so much the house itself that you pay for, as this list shows. It's where it is.
What makes a mansion? I put up a list last time the mansion tax was being talked about. But for amusement's sake, here is another compiled by an irritated London house-owning friend.
You can see why the whole thing makes Londoners so furious. But the list also does a good job of proving the point that you don't pay much for houses, but you do pay (or not) for the location of houses.
Mansions in London owned by rich, evil capitalists who don't pay enough tax and need to be dealt with accordingly:
Perfectly average homes in the rest of the UK owned by ordinary people who shouldn't be taxed any more than they are already: