Thank you for this thread.
We year-deferred for our August-born son four years ago. He had severe receptive language delay - the LA's recommendation had been that we apply for the default year but also apply for a statement. Thankfully though, the paeds, SALTS, head teacher and nursery manager all supported our wish to defer.
It has worked out beautifully, he is in year 3 now. As pointed out above, the only problems threatened are ones created by the system itself. We did have an arrangement with the LA that he would have to stay in the year he was placed in. Our secondary has indeed proceeded to become an academy though thankfully it has delegated its admissions function back to its LA. And as Tiggytape points out, that does indeed mean that we cannot leave the area where we live.
A good diagnostic for us is "given the risks of some Kafkaesque bureaucratic horror story at the secondary school transfer point, and given that you have had to stay within your LA and that has slightly held back your DH's career, was it worth it?"
To which the answer is a resounding "Yes, I don't have to think about that even for a moment".
I have seen the children who were "just summer-born" get over it in their default year. But it was different for DS. You can pay for one-to-one, but you cannot adjust the peer group and you cannot pay for the extra time your child needs to develop, now matter how skilled the teacher. What DS was given was the extra time he needed in the right environment. As his abilities have evened out, he has had peers at a similar level around him. That wouldn't have happened in the default year. At most, he would have been babied.
[Tiggytape, the teasing thing hasn't been an issue so far by the way.Also, on the weighing scales, even if it happens, it would be nothing compared with the alternative I think].