There's an up-to-date piece by Full Fact showing how grammar schools with catchment areas worsen segregation and even lower attainment overall. Chris Cook did some research on this for the FT and it links to his charts. Pupils in secondary moderns have fewer opportunities to do single sciences, etc.
Richmond residents (parents) are overall three times as likely to be educated to degree level compared to Dover. That may explain why our comprehensives still have a lot of high achievers despite a large private sector. The grammar system seems to lead to complacency in provincial towns - I checked A-level subjects for Kent grammars and the numbers doing Latin, Music, languages or other such subjects are minuscule. A surprising number of U-grades too. So while they have much greater opportunities, they don't seem to make the most of them.
I can't see how such changes are going to get past the Commons let alone the Lords as there is lot of disquiet among Conservatives MPs. Gove may also join in a rebellion (his former adviser Sam Freedman is against new selection - how can that work with an academised system of autonomous schools?).