Scrap them all and get parents working together to pressure the government to work harder so that everyone has a decent school choice.
I hope you include all selection, including faith schools, arts, sports etc? I believe we should either allow a full range of selection or allow no selection at all. I wonder how many of those opposed to grammars are happy due to being able to meet a good faith school criteria? Double standards I think!
Unfortunately, until the educational establishment can agree between itself how to improve, politicians are just as impotent as the general public. All we ever hear is that "more money" will solve it. Bullshit. More money will just be wasted doing the same things and lining the pockets of the people involved. We've had so many different "incentives" since the original idea to scrap grammars. We've had subject specialist schools, we've had massive spending on new facilities, we've had parental choice, we've had academies, the syllabii have been changed many times, exams have been changed, coursework and modules introduced and then scrapped, subjects introduced, subjects scrapped again. The truth is that no one knows how to improve the UK education system. Even with political pressure (and more money!), there are no guarantees that whatever new initiatives are introduced will actually work. Every time a new initiative is rolled out, the poor pupils are mere guinea pigs and many suffer as a result when the new initiative doesn't work out as planned!
Like everything to do with the school system, it seems very individual to the school as to what works and what doesn't - that will depend on the type of pupils they have, the staff they have, the head and management team, - lots of variables. What works in school A may not work in school B. Sometimes a strict head imposing discipline seems to work, but in another school it may fail - that's because of the different types of pupil, different social backgrounds, different parental support, etc.
We've had over 50 years of "progression" since most grammar schools were scrapped. If comps were as good as some people think, we wouldn't be having this discussion and the remaining grammars would have been closed down. 50 years later and still the educational experts are no closer to agreeing how to solve the problem and make all comps good! So, let's all blame the grammars instead to divert the blame. If anything thinks that scrapping the grammars would automatically improve the comps, they're deluded!