The panel only have the information that you, the parent, submit about your child. That will include the Head's appeal summary sheet, but no more than that. The school does not submit any information about your child directly.
The only caveat is that, if the LEA rep sees a significant discrepancy or an issue in the case documents that needs more information, s/he can ring the school to discuss it. That happened in a case I was involved with last year where the Head had clearly made a rather large error in the predicted SATs on the summary sheet, and it was pointed out by the parents in their letter to the panel.
I should mention that in that case the LEA rep's investigation strongly backed up the point that the parents made in the letter, and the rep was most forthcoming about it, and in an entirely positive way. Indeed, he probably won the appeal for the child concerned.
The LEA reps may be the "opposition" at an appeal, but the experience of Bucks parents is generally that they are absolutely fair.
P.S. Guest55 has, as ever, nipped in and left a pithy response whilst I type my plodding essay! Still, you get two-for-the-price-of-one, which seems to be fairly standard these days.