The school has the right to demand top up from the LA for EHCPs (and even without EHCP if there is SEN that needs extra resources) However, with the delegated funding there has been a big drop in the Pot which these very popular schools receive for children with EHCPs, and if you remember that now the schools are told they have the money already allocated in form of delegated funding, they have to go through all sorts of hoops to GET the top up funding. It can take many months or years for the school to prove it needs extra resources and for the council to actually provide it with the money.
It is now illegal for academies to refuse children with EHCP's if the parents name that school, and there is no obvious reason why it is an unsuitable placement. So obviously the "outstanding" academies will get all the children whose parents would historically have wanted the best for them. If your child has an EHCP and you asked to name a good or an outstanding school, a school which has no particular track record or a school which is known locally for its provision, which would you pick?
In a sense the school becomes a victim of its own success, too many applications, and then they really cannot provide the resources, for the reasons I've outlined above, since 2014 the funding on EHCP is assumed to be delegated, and the bulk doesn't come with the child anymore.
I agree that the answer is for all schools in the area to have a good reputation for SEN but how do you achieve that? Pump extra funds into the schools which receive less applications? Change the law on parents' choice of school? Give pep talks to parents in advance explaining why the obvious choice is not the obvious choice?
I've been through all this; in our case the school didn't want us without extra money, which the LA wouldn't provide. They were completely full and had many children with EHCPs and additional needs (which is why we chose them, that and the Outstanding) It was a game of cat and mouse, playing out the legal implications that the school could not turn us down and had to provide the resources outlined in EHCP, with or without the money. Council was very keen not to have this battle and persuade us to go to another school, but I can see how difficult it is for the school, and by extension for the council, when parents are set on a particular school which is completely oversubscribed with SEN.