Indian Bicycle Marketing: Boris Johnson special

From The Times 30 May 2014:

Older people who sell up their family home to downsize would be exempt from paying inheritance tax on their profits under plans being put forward by Boris Johnson.

The London Mayor wants the over-65s to free up large houses and move to smaller retirement flats or homes to help solve a critical housing shortage in the capital. Many elderly people are put off downsizing when their families have left home because they could gain a big profit which would be liable to inheritance tax when they died.

[NB: the reported logic is total shit; inheritance tax is on the total value of somebody's estate, so if a Poor Widow in a £1m home sells up and buys a £200,000 flat and dies in that flat with £800,000 in the bank, the inheritance tax is exactly the same as if she's stayed in the £1m home. But there is a germ of a good idea in there - i.e. simply exempt everything except land from IHT etc.]

From The Evening Standard 4 June:

Boris Johnson today called for rich foreign investors to have their council tax hiked to 10 times the current rate if they leave their homes empty. The Mayor hit out at overseas owners who buy up property in some of London's most exclusive areas to keep them as "a bank balance in the sky".

From The Evening Standard 3 June:

Boris Johnson today told Brussels officials to “take a running jump” after they demanded Britain impose higher taxes on people in expensive homes.

The idea of a new “mansion tax” — which was dubbed a “tax on London” — was in a European Commission document sent to Chancellor George Osborne containing a list of economic policy suggestions. Treasury sources were cool on the Brussels demands — but the Mayor of London and Tory backbenchers reacted with fury.
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There is no real coherence to any of this, in particular, how do you define "empty"? A four-bed home with nobody in it is clearly empty and a four-bed home with a family of six in it clearly isn't. But what about a four-bed home with one person in it? Isn't that nearly empty? What if that home is divided into two two-bed flats with one person living in one of them - how are the two homes any emptier than the four-bed house next door with one person in it?

At the levels they are talking about, this is not intended particularly as a revenue-raiser, it is about encouraging efficient use of land and buildings (and hence of everything else as well), so why not just have higher ad valorem council tax on everything use part of the proceeds to scrap IHT completely and dish out the rest as a flat cash rebate; if Poor Widows are your thing, then share it out among the over-65s AFAIC, so a genuinely Poor Widow in a council flat comes out miles ahead of the game and a Poor Widow In A Mansion just about breaks even?