I wish it were easier. I posted a few days ago asking for help with appeals for this, but got no responses, presumably because it's so rare.
I have a pupil who is working about 3-4 years below her chronological age, and is behaviourally and emotionally 1-2 years behind. The mum has been asking since reception to hold her back a year, and every professional involved with her agrees that would be the sensible solution. The child herself wants it, and prefers the younger children. She's just moved to a new village, where there are no places available in her year, but there are in the younger year (in fact a split class with the year below that, which would offer her the chance to work with children somewhat closer to her academic age). New school, fewer social problems moving years. She's already having problems socially anyway, given that she's at such a different level. But applications for deferral have been refused, with no right of appeal. It's frustrating, because she is being pushed on, when she is getting almost nothing out of being in her current year. She'd have problems whatever year she was in, but she has the maturity now to get something out of the phonics teaching that happens in the earlier years, that she missed back then, and the gap woudln't widen as quickly as it would if she were kept in the current year.
There are lots of children that would have benefited from a later start, or the chance to repeat a year, but it just seems so impossible.
I only know of one child who managed it, but he'd had meningitis in Reception, missed quite a lot of school, and had hearing problems, so he was allowed to repeat Reception the following year, and stay down for the rest of his schooling.