I think you will find that the panel will be aware that these curriculum levels will place your child firmly in the top 25%; I don't think you need to prove it statistically, blow by blow as it were.
She is currently working at level 5b maths, level 5b reading and level 5a writing. She is the only child in her year group at level 5a for writing
Presumably you will have a letter from your primary school head confirming the levels and it would be helpful if he/she could draw attention to the fact that she is the only one at 5a for writing in a class where 17 passed the 11+. English writing has by far the lowest numbers at Level 5 overall for all tested SATS subjects - your school would have the figures who achieved it in previous years with a comparison against national levels.