DS1 was born early July. Academically he may have benefited from starting a year later. Socially and based on personality? Nope, he needed to go when he did. He's a juggernaut. Has been since he could crawl. He would have flattened the children in his class had I kept him back a year. From a social skills POV, he does well being one of the younger ones. He's extremely articulate and when he was younger, got frustrated with conversations with younger children. That's settled now he's older. His boisterous nature means that he's able to bounce around with friends that are almost a year older than him, and as such a bit hardier.
From an academic perspective, well he's not really, so I think that he would have always struggled. The school are amazing, and he gets support where he needs it.
It's worth noting that if you defer, they will usually go straight into year one, which was the case with a girl in DS1s class. For me, that's too big a jump from a preschool environment. Reception acts as a bridge between preschool and the National Curriculum. Reception still comes under the EYFS, which is the curriculum used by preschools. It's in this year that they get the chance to learn how to be at school and to be honest, they spend most of the day playing. Year one is much more academically focused.
From personal experience, I'm a late late August baby. I started a week after my 4th birthday. Apparently I was ready, so off I went. Any struggles that I had through my school life were unrelated to my age.
Ultimately, you know your child best. Would you consider part time reception? Your child may surprise you by wanting to do more.