I have massive sympathy for these parents - I also think it's a very fortunate minority of SEN parents*, as so many more just don't have this option - for me, even at the really cheap local private school, it's £9k a year per child, and that's way beyond my means. I'd support some kind of rebate (trying to avoid saying 'exemption' to avoid confusion) where there is evidence of need (probably an NHS diagnosis of some kind).
And actually I wouldn't do VAT anyway. As I think private education is bad for society, like I think the national lottery and smoking are, I'd put a specific tax duty on them (like the duty on fuel, gambling and tobacco). Then you could include SEN in it.
Then legislation would be drafted by people who know about tax and education, consulted pn, refined in the House of Commons and the Lords and implemented. It's a cleaner, more transparent way of achieving policy aims and would stop people on mumsnet arguing about VAT when they don't understand it.
I'd then have a policy which created state schools suitable for academically able children with autism and ADHD, which would be much cheaper than current special schools while meeting needs. And I'd train primary teachers on spotting ADHD and autism in girls and meeting their needs. And voila, I have solved all society's problems! Go me!
Please note, this is an unfunded policy...
*'fortunate SEN parent' is arguably a bad choice of words. Hopefully people will know what I mean. Also, neurodivergent children can be amazing, but parenting them can be challenging. Also I know SEN is much broader than autism and ADHD but that's my experience so that's what I think about.