@nc100: "I'd be worried about them managing to integrate further down the line. If they go to school for the first time aged 9 for example, there are surely things that they will be expected to understand and conform to that won't be necessary/taught at home."
Insofar as school is well suited to a child's needs, the child will find it easy to adapt to school. It shouldn't take five years to prepare a child to cope with the academic, physical, and social environment of Y5. My older child did start school for the first time at nine, and found the adjustment trivial. The other kids weren't really conscious of the fact that she was new to school, though of course they knew she was new to THEIR school. Within two weeks she knew where everything was, could always remember to put her hand up if she wanted to use the toilet, formatted her homework in the approved style and remembered when it was due, and had found a way to navigate many of the social restrictions which her peers sought to impose on her.
For a four year old, much of this stuff IS really difficult. I reckon this is why people conclude that starting school is difficult full stop. But it's difficult because they are four, and too little to cope with some school challenges without considerable adult help. For a nine year old with more than twice as much life experience, who happens to be "school shaped" (round peg in a round hole: sociable, willing to conform, able to sit still, interested in academic topics), it's quite easy.
On the other hand, there are children who are not well served by the school system. They aren't "school shaped" and will never thrive in such an environment, whether they start at four or nine. My younger child is like this. She enjoys peace and solitude for most of her day. She is active and craves novelty. She loves tinkering with things and dislikes writing. She has a learning disability which means she hasn't been able to grasp some fundamental concepts until years later than her age peers. Her emotional and social development are likewise delayed.
Unlike her sibling, if she'd started school aged nine she wouldn't have adapted easily. In fact, I doubt she ever would have adapted sufficiently to be thrive at school. But the same would have been true in spades if she had started at four. If it had been necessary for her to go to school, I'd rather have sent her at nine than four. By nine, she had developed the strong self esteem which is the consequence of having had reasonable expectations imposed on her in an environment which suited her well. In short, if she started school and hated it, she might have been in a position to realise that the fault was not with her. She'd have had five happy years of suitable education under her belt. What's more, by the time she was nine, her special needs were better documented and I could have fought for her needs to be met at school from day one, rather than after several years of floundering.
Luckily I don't foresee ever having to send her to school. I'm glad I don't have to try to prepare her to fit into an environment which is all wrong for her. I am confident that she'll land on her feet eventually and do well in life as an adult, because the wider world is not very similar to school. She'll be able to find a job which is right for her. She already has friendships which make her happy, and is developing the social skills to deal with a range of people in different situations.