This is the case in Ireland and the past two decades have seen an explosion of "redshirting" and later starting. Basically everything Tiggy describes is spot on.
When I was young back in the stone age, about 50-50 of summer borns (thats June/July/August) chose to start at 5. But there's been an arms race now for years. It's normal now for middle class kids to start at 5, not 4.
Irish parenting forums have posts from parents with children turning 5 in October seriously considering keeping them back until they are almost 6. And other posters encourage them to - I mean why not, it's always best to be the oldest not the youngest (you see this all the time on boards.ie or the old magicmum site for anyone curious).
So you not infrequently get classes which contain a child who just turned 4 in August and another child who will be 6 in October. 6 year olds are experiencing a curriculum designed for 4 year olds and teachers have to try to balance the needs of children almost 24 months apart.
And the early years foundation stage is not even the most difficult time for this!! Think what it's like ten years down the road when puberty hits. Kids tower above other kids. Imagine being two years ahead or behind everyone else with breasts and periods - it is Not Fun. Or fair on the kids.
I used to wish the British system had a bit more flexibility but having seen the Irish arms race at first hand, I much prefer the strict criteria here. It quite properly keeps the expectation on teachers and schools to cater to the children's individual educational needs, instead of expecting parents to perform assessments no one is capable of.
Maybe the starting age should be moved back to 4 by April. Yes, that's still rough on the March kids but at least they have five extra months on the August kids today which can make a big difference. But any move to introduce flexibility or choice should be strongly resisted - they are buzzwords that sound alluring to parents who are educated and wealthy enough to exploit them, but a disaster for the education of poorer kids.