[michaelt]

  Why on earth would you buy in London in these
  circumstances, why not put you money in an ETF?

Because you're a money-laundering oligarch (or a upper class person in an oppressive regime, worried the state will appropriate your assets) and if you're spending £10 million or so on a property, you can afford to conceal your identity with an intermediary in Jersey, Guernsey, BVI, Panama...

[YorkshireSeason]

    conceal your identity 

You can also conceal your identity when buying an ETF. An EFT almost certainly has a higher ROI than empty property.

And if you are investing via intermediaries, then they can handles the letting process so as to maximise the ROI.