Many parents use the private sector for different reasons. For example, some parents choose to send their quiet, anxious child to a private school, a child who could be displaying SEN traits. That child copes perfectly well with smaller classes and tailored education without an EHCP. They would fall apart in the state system.
The VAT is going raise a tiny amount (even looking at their optimistic figures). Any tax raised isn’t going to fund any state SEN provision. It’s been allocated to new expert teachers, teacher and headteacher training, work experience and careers advice, language development in primary schools, ofsted reform, new nurseries and mental health support for every school. All this is going to be funded by the 1% of the state education budget estimated to be raised by the VAT. Not a single penny is allocated to SEN support.