There have already been moves to change the entrance criteria for grammar schools to prioritize kids from lower income families, and I would be surprised if we did not make more concerted moves in the years ahead in this direction. because if we do see a big increase in the number of “prep school kids” going to grammar, it is going to make all the vague burbling about the “marvelous social mobility” offered by grammars look pretty hollow, and will prompt some demands for more far reaching changes to the intake rule.
(In much the same way as, many universities further shifted in the direction of “contextualizing” state vs private school applicants after it became clear that many private schools blatantly inflated their exam results during Covid, much more than the state schools did. I am not trying to bash private schooling, my elder DD goes to a non-UK private school, but frankly it was awful behavior, and I am not surprised that there has been a bit of a backlash. I can remember some of the anger among the state school parents in the UK who I talked to, when the exam inflation stuff was in the news. It is possibly one reason why the VAT thing has been so popular among state school parents, to be honest)
It does occur to me, though, that the private sector may be missing a trick here. grammar schools actually get less funding per head than other state secondaries due low levels of PP, their facilities are similar to other state schools, their classes are usually big as they let in all the applicants they can. My DD goes to a private school outside the UK which is like that (we chose private because we wanted a bilingual school for my English speaking DD, not for little classes and marvelous music and sport) which is why it is affordable. Looking at the fact that many private school parents are happily turning to grammars as an alternative, it looks to me like there may well be a market for no-frills private secondary schools in many cities, i.e. with normal class sizes and facilities, and entrance exams to ensure intakes which are able to keep up with a more “stretching” curriculum. I am surprised that there has not been more conversation in the UK sector about providing for this.