I think funding is not quite that simple.
Statemented kids come with money. But there always have been very few statemented kids.
The rest of the money does not come from the number of kids on the SEN register.
Each LEA has a certain amount of money to spend on education. They then decide the funding formula they will use to split the money between all the schools.
AFAIK my LEA decided the funding formula in a meeting with all the HTs. And the funding was based on head count and cohort banding. We have some banding thing where schools are on a band of A to F depending on their cohort. So not just so simple as to how many FSM kids, but also taking into account the demographics of the community. Ie average education of parents and average wage of parents etc, etc.
So the HTs between themselves have agreed that schools in band A gets X per kid and schools in band F get Y per kid.
Some money is also designated SEN budget. But I'm not sure if it's related to how many kids are on the SEN register, or school size, or school banding......
Now if the total pot of money the LEA has to spend on education shrinks then obviously each school gets less money. But if the total pot doesn't shrink but the number of kids on the SEN register does - then next year the funding formula will be different, but the HTers will still try to split the money as fairly as they can......
So it is the education budget shrinking which is the concern, not how many kids are designated to have SEN.