This might be of interest. Bexley Council says the Headteacher review panel received 115 requests from parents this year for their child's score to be reviewed.
Three candidates were successful, two being promoted to selective and another being offered a re-test.
No requests were granted on the grounds of the pupil being unwell during the exam. The council's view is that the child's parents should have asked for them to sit the test at a later date.

So, there you have from the horse's mouth, as they say. I am assuming that Bexley does also publish this advice in advance, at the venue etc?

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Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.Groucho Marx

I suspect that those conditions about illness are not always well publicised given the number of parents who appealed on health grounds - fruitlessly. Council refuses to say what factors were in play with the successful regrades citing confidentiality. Parents in my experience also contend with primary schools that are opposed to selective education but grudgingly hold the 11 plus exams and offer little support/advice. My son was told after he passed by his primary teacher: “well done, you’ve just qualified for five years of needless pressure at a grammar school!”
Both my kids are at grammar and are doing well enough thankfully.