We have one in independent and three in state (two secondary, one primary; eldest went to the same state primary before switching to independent - we were two years into what ended up being a four year process to obtain an ASD diagnosis for her and, after she got lost in a tiny state primary couldn't see her coping with a large secondary).
She has just started Y12, so we were at least able to go into this with our eyes open and fortunately she has/kept an academic scholarship, otherwise we'd have thought twice. After leaving (a lovely) primary (it just didn't work for her in Ys 5 and 6), she has thrived at her tiny independent school, which isn't about academic achievement but success being individual.
We had a place for her in the sixth form of the secondary that DC2 and 3 attend but didn't want to unsettle her, so have bitten the bullet and paid. As we have for everything arising from her ASD. I appreciate we are fortunate to be able to do so, but it does mean missing out in other things. But that's our choice.
The impact will come later, as people defer entry to independents or leave at particular key stages. As someone who lives in an entirely selective county, it's way more likely to affect the Y6 children moving to Y7, with increased competition for grammar places, I would expect.
I don't have any issue with the policy per se (and voted Lib Dem so knew it would be coming in), as state schools need the support. I do, however, have an issue with how it's been implemented with no concern for the children. There are so many children with mild/moderate SEND at DD1's school
and they are the type who will just get lost within the system if they have to move. Which benefits no one.