I don't understand the principle.
You can't have ALL schools being selective by definition. If you select the top 25% into one school, there must be 3 non-selective schools nearby to cater for the rest.
Forcing grammars to take in a proportion of low-income pupils is good, but could be tricky if they have to drop standards for them. You could imagine them being bullied or marginalised in the school ("you're only here because you're poor", "poorer children aren't as clever" etc). I like the principle of selecting on ability rather than house price though.
And how does "'here will be no “return to the past” of mass 11-plus tests to ensure that schools do not separate children into “winners and losers”' work? How else will you decide who goes to these new grammars?