So a basic chunk for all children, plus up to £6K from schools Sen pot, plus more from LA if needed
The "more from LA if needed" generally only applies if children have EHCPs. A few operate a high needs funding system whereby schools can apply for extra funding without having to go through the EHCP process, but this is unsatisfactory for lots of reasons - notably that the funding can be taken away at any time without parents having any avenue of appeal or enforcement.
The £6K is a notional sum based on a complicated calculation; in practice schools receive a sum which assumes that, whilst some children (including those with EHCPs) will need support amounting to £6K, some won't. Schools will tell you that in practice this calculation means that they don't receive anywhere near enough to support all children with SEN, and it's a positive disincentive to them to take children with a high level of need.
However, most materially, none of this has anything at all to do with the bringing in of EHC Plans instead of Statements. There were similar delegated funding arrangements in place well before the Children and Families Act was passed. Funding arrangements are reviewed every year and are a completely separate process. In many respects, one of the advantages of an EHCP from the parents' point of view is that it takes your child out of that uncertainty, because by law they have to receive all the support in section F come what may, and irrespective of funding arrangements.