Holding back a year does depend on your LEA. Some (Hertfordshire) are really easy. Some are almost impossible with out some sort of SEN.
Anecdotes about how summer Irma were fine 30 years ago, some of my best friends at oxbridge were summerborns and didn't have a problem, my summerborn child is just fine etc etc aren't all that helpful.
Statistically, at a population level, summerborns in England are disadvantaged. There is no doubt about that. And that disadvantage persists throughout education. By the time you get to university it's small, only 1-2% difference. But it's still there.
And it's not all about academics either. Summerborns are statistically more likely to suffer from self esteem problems and depression. And summer born boys have far higher rates of ADHD diagnosis.
The flexible schooling for summer borns Facebook group is very useful, but be warned, it is heavily biased, and you can easily find yourself getting trapped down the "not deferring my summerborns is tantamount to child abuse" rabbit hole, which is also not that useful! Worth a look, but try and keep a balanced view.
Fwiw my DS is a summerborn, and is in reception with his cohort. That took an awful lot of handwringing and questioning and worrying. And I'm still not confident in my final choice for him.