A This issue continues to be sensitive in some quarters and inequitable in others. Much depends on your status within the school. Your task, whatever your status, is to demonstrate to the headteacher and governors that involving you will help them make the best use of limited funding.
You may need to do some homework first. Start by finding out exactly what system your local authority uses for distributing funds for SEN. Systems vary across the country, as do the principles and terminology used. This information and details about the funding each school receives are publicly available from your council. You can also calculate from your own knowledge of pay scales and staffing roughly how much your school currently spends on SEN in terms of human and physical resources.
This research, combined with your knowledge of the school's profile of need, will have set you thinking, revising your department's development plan and providing a range of options. This would be the point to ask for a meeting with your headteacher and SEN governor. At worst, they will ignore you, at best they will be as interested as you are in the possibilities for improvement you open up for them.