"I struggle to understand why Britain continues with such a young school-starting age when the evidence (maybe particularly for boys) suggests that kids do better if allowed a later start to formal learning. Threads about 'teaching my 2 year old to read with flashcards' just make me despair tbh."
Yes, this, exactly!
I can say that my winter-born DSD in Scotland also struggles with school. She started P1 when she was 4.5 and it was way too young. She is making progress, but she is consistently behind - not only behind her peers, who are all older than her (she's the youngest in her year), but also about six months behind her so-called "reading age", according to various "learn to read" publishers. It is not just reading and spelling she struggles with either - she still does not have basic sums in her head - she is nearly done with P3, and still doesn't know 8+7 off the top of her head.
During P1, she may as well have not been there. None of the material she went over during the entire year stuck. It finally began to stick about halfway through P2, just before she turned 6. I believe that is when a lot of children are more developmentally ready for a formal school environment. You can close the gap between summer/winter borns and the older kids, but the most practical answer is not by fudging the exam scores - it's by putting children into school when most of them are more developmentally ready to handle it.
In my DSD's case, yes, my DH and his exW could have deferred her. But the staff at DSD's nursery said she was ready for school. And so, they trusted the nursery. By the time they figured out that DSD being able to read/write her first name, but not her last name, might not cut it for P1, it was too late - she was enrolled.