London has also seen a huge glut of (supposedly) high-end property come online. Builders are struggling to sell, so they're now starting to offer discounts on the new housing stock [1]. If what the article says is true, I would be surprised if the same scenario didn't play out in London.
[1] https://www.ft.com/content/f6692d4e-8327-11e6-8897-2359a58ac...
So I guess this is a case of convergence, higher priced housing is dropping and lower priced housing is increasing. This effect will probably become more pronounced if interest rates start to rise.
1: http://uk.reuters.com/article/uk-britain-eu-banks-idUKKCN12M...
2: https://www.theguardian.com/politics/2016/oct/22/leading-ban...
This isn't too surprising, since I think London rents were seriously testing what the market would bear - plenty of chatter about London becoming completely unaffordable.