I would advise getting a doctor's confirmation of the illness, dated as close as possible to the day of the test. Hopefully you won't need it but you will rest easier if you have taken that precaution. Also make sure your primary headteacher is formally advised that your daughter was unwell. I don't see how you could regret getting those things in place just in case. Good luck!
I'm so sorry for you both! Is it worth taking her to your GP so that you can get written confirmation that she's poorly or on medication, in case you need evidence at a later date?
this won't help - if she is ill then she must not take the test, the assumption is that she is well enough to do so if she turns up. Evidence that she was not well will not help at an appeal