Rather than offering places based on living in a catchment area, surely grammar schools should simply admit by highest ranking? This would prevent local schools from losing all their bright pupils as some would not get into the grammar of it was purely based on ranking score. For example in Barnet, QE boys purely admits on highest ranking and all the local schools are doing really well compared to the average secondary school in England.
In addition, it would make Grammar schools even more meritocratic as you don’t need to live in the (often expensive) catchment area to get in.