I think what they are saying is that they have made the judgement that any child who attends mainstream school will have their needs met from within delegated/enhanced resources and therefore, by definition, will not require a Statement.
This is because the current threshold for a Statement is that the child has SEN to a degree that the nature of provision necessary is beyond the resources of MS schools in the area:
"8:1 Once all the advice requested for the statutory assessment has been received, as described in Chapter Seven, the LEA must decide whether to draw up a statement. The LEA may decide that the degree of the child’s learning difficulty and the nature of the provision necessary to meet the child’s special educational needs is such as to require the LEA to determine the child’s special educational provision through a statement."
8:2 The LEA will make this decision when it considers that the special educational provision necessary to meet the child’s needs cannot reasonably be provided within the resources normally available to mainstream schools and early education settings in the area. "
It's often overlooked, but this is a contextual condition. In other words, the same child may need a Statement in one area, but not need one in another area, because different areas have different provisions available as 'standard' and different funding agreements:
"8:14 The decision as to whether to make a statement should be determined by the child’s identifiable special educational needs in the context of arrangements for funding schools in the area."
Working backwards, from Statements to Statutory Assessments, an LA should not carry out SA if they believe that a Statement is not 'probably' necessary:
"7:4 An assessment under section 323 of the Education Act 1996 should only be undertaken if the LEA believe that the child probably has special educational needs and that the LEA needs or probably needs to determine the child’s special educational provision itself by making a statement."
Your LA, from the sound of it, have decided that the needs presented to them in requesting a SA are needs that can be met by the resources available to (that is, not necessarily already held by the school, but obtainable by following a process to get extra funds) the MS school your DD attends.
I suppose an analogy may go like this:
It's 2009 and you are diagnosed with a medical condition known to respond to a particular medication. Due to funding constraints and red tape, you can only get that medication from a specialist. So, your GP, knowing you need this particular medication, has to refer to the specialist, who will then see you and say 'she needs this medication'.
Now, roll on a few years (in our case, we're rolling on to the new funding system in SEN):
It's 2013 and you are diagnosed with a medical condition known to respond to a particular medication. Everyone tells you that you're going to need to see a specialist. But, when you go to the GP, they say 'oh it's alright, I know what medication you need, here it is.' Since 2009 they've changed the system and you no longer need a specialist referral to get the right medication.
So the situation has changed.
I'm not saying that your DD doesn't need SA. It's your right to appeal. I'm just saying that if the judgement they have made is that she doesn't need support beyond that available to her school, then they have made the right decision.
We as parents like the accountability and legality of Statements, but they aren't the criteria for assessment. The criteria is a strict needs based analysis. If the school can access the necessary support for your DD through SA+ then she doesn't need a Statement.
What you could do, is have a think about what support she gets now, what support you think she needs, then see how much of that support you think is beyond the available resources of her school. That might sharpen up any appeal you make.