Agree with Hollow. Prices have actually gone up in the cheaper boroughs, which are in a completely different housing market than Kensington, Islington etc.
In recent Rightmove data which shows changes in asking prices by borough, Lewisham asking prices went up around 2% between Sept 2016 and Sept 2017. Hardly sign of a buoyant market but at least they are not decreasing. Of course the bigger question is what is happening to the prices these properties are actually selling at - not aware of whether someone has done a recent study on this.
Source - http://22123-presscdn.pagely.netdna-cdn.com/news/wp-content/uploads/2009/07/boroughs.jpg