It's hard to influence individual school SEN policies as a parent without institutional backup of one sort or another. How hard it is varies from school to school, and governing body to governing body.
I was a governor for a a few years. One of the most frustrating things I found - particularly on SEN issues - was the strategic impotence.
Addressing the workaday problems with implementing SEN provision (like poor-quality IEPs) - are well beyond the scope of what a governor is supposed to do. But a lot of stuff I considered to be strategic - prioritisation of SEN issues, poor allocation of resources, inadequate staff training, over-reliance on unproven woo interventions - was classed as 'operational' and therefore out of bounds.
On the other hand, rubber-stamping vague, catch-all policy documents offering no meaningful strategic direction of any sort? Yep, these were 'strategic.'
Like star said above, as a governor you can ask why the school has chosen to spunk its entire SEN budget on a library / mock-Ofsted inspection / set of 6 Thinking Hats. But at my school's GB, there was no expectation that this 'challenge' would result in a change in outcome, or even that it should.
I'd hope that it's different at other schools - but I wouldn't be sure, particularly for those schools that are dementedly focused on changing their Ofsted category. In the end, I stopped being a governor because I had major problems with what the school was prepared to do to get out of its category (another story, not a pretty one).
If you're looking to change the policies that are being implemented on or for your child, you might be better off starting at the frontline and working upwards. There are classroom teachers and TAs out there who haven't been completely beaten down by the system yet...