I'm not sure a fire sale of such homes would have an effect but presumably you've done the maths?

If concern about the housing bubble is the behind all this then I share those concerns. Personally I think London will be finished unless affordable housing keeps pace with demand. I think the simplist way to do this are to create rent controls and proper long term tenancies like in most parts of the world. This would presumably also reduce house prices as it would effectively lower the value of the land for buy to let investors.