From November 2018, if delivery falls below 25% of the requirement, the presumption in favour of development as set out in the NPPF would apply automatically.
No it cannot without primary legislation. The courts have held that a weighted balencing exercise still needs to be carried out against the development plan and other material considerations. A matter of policy alone cannot make a material consideration immaterial.
The effect of this application of course would mean that every single planning application in London, apart from those affecting Heritage Assests or Green Belt, would have to be approved.