Another school here that if anything underpredicts for mocks (and has a reputation for doing so). If only 18% of students get their predicted grades in A Levels, isn't it likely to be similar in GCSEs too?
I'm not contemplating requesting any reviews of marking for DD but there again she was someone who 'got lucky' with two of her highest grades, just scraping them.
Got caught up in doing it for DC1 (who was very close to higher grade boundaries across four subjects at GCSE), fuelled on by MN posts (!), and I ended up circa £200 out of pocket, with DC1 very disappointed because his grades didn't change at all (he lost a few points along the way). It was also very important for DC1 to get better grades because sixth form place was dependent upon it.
It's also yet another way in which the reasonably well-off can throw money at helping their DC (as with tutoring), in a way that those who are struggling cannot. I know some schools do absorb the cost of reviews of marking but based on my own experience, it's certainly not all of them.