In hindsight I wish I had. DS is a July baby, and is very bright and academically able. When we asked his nursery teacher if we should, she said "I wouldn't, he'd be bored."
Thing is, as bright as he is, he's also very sensitive and emotional. He coped well in Reception, but in Y1 he transformed from the sweet, happy, friendly little boy he'd always been to a very sad, anxious child who lacked confidence and self-esteem. So sudden and dramatic was the change that I actually went to our GP to rule out anything medical.
He's also been periodically bullied throughout school, and the school hasn't been great at dealing with it. Of course this may have happened anyway, but a year later he may have had more resources to cope with it.
Finally, there are two and a half years between him and his big sister, but only 2 school years, so he'll be sitting GCSEs at the same time she's taking A Levels. Another year would've meant we don't have to split our focus in supporting them.
So yes, I wish I had held him back a year, and regret not doing so.