Earlier quoted context omitted.
If the planning laws were changed and a couple of thousand were built then the market prices would come down. Developers would still turn a profit because the cost of building is ultra dependent on the cost of land in London, which if the planning laws were changed would change.
Buy prices are already slighly down compared to 2 years ago, something to do with Brexit. There are literally properties being built everywhere. Just take the overground to any direction and look around you. It doesn't make prices go down. There are too many people and too much money.
Prices are also down because of tax - sharp tax; which is a good thing in terms of dealing with speculation.