Not really - in London (and the rest of the UK) over the last few years large corporate landlords have been building huge developments of thousands of apartment

Not really - in London (and the rest of the UK) over the last few years large corporate landlords have been building huge developments of thousands of apartments, called the "build to rent" sector. It's unlikely all of these developments would have went ahead without them. Even in the 'normal' development sector, landlords buy a significant proportion of the apartments available from new build developments. I would estimate that 30-40% of the apartments in my building were bought by landlords ahead of the development starting. This adds liquidity to the market and allows developers to build knowing they are taking less risk on selling them when the building is (more) complete.