I'm all for demonstrating about the whole sorry mess, but I'm not sure this demonstration is targeting the right thing. They say they want it to be made illegal for provision to be removed from a statement when transferring to an EHCP, unless that removal is supported by professionals. In principle that's absolutely right, but what do they mean by professionals? If they mean people like LA EPs, or schools, it is going to be child's play for LAs to get the required professional opinion.
I think it would be more productive to demonstrate in favour of making the law much easier to enforce, e.g. so that LAs would be penalised if they don't comply with time limits for the EHCP process, or they don't produce detailed and specific provision in section F, or if they fail to make it available - i.e. so that it is not left solely up to parents to enforce their children's entitlement.