LONDON — When Andy Burnham enters Downing Street in less than a fortnight, the Labour veteran will bring with him a plan crafted by one of his party’s newest MPs. That blueprint for government was prepared by Miatta Fahnbulleh — a Burnham ally who shares his conviction that it will take radical action if a new administration is to rejuvenate the ruling Labour Party, turn around Brits’ quality of life, and hold populism at bay.