I have never for one second regretted deferring dd1 and ds (both Feb birthdays, so started school at 5y6m). They got an extra year of nursery, and have done fantastically well at school, I am sure because they are that little bit older and more mature.
Dd2 is an end November birthday and we didn't defer - and now I have occasional twinges - she's much further behind that the other 2 were at the same stage (p2) and only now getting to grips with school - tbh she would honestly be happier at home with me i think!! (but she's always been clingier!)
The only person I know with regrets is also someone who didn't defer, and she went on to defer her younger ds several years later.
As a secondary teacher, I think it is a good idea. At 11/12 I think it especially applies to boys ime, but the kid who hasn't done their homework/has forgotten their folder/goes to the wrong classroom all the time - that almost always an 11.5 rather than a 12.5 year old. They also cope much better with the change to a more independent style of learning.
I'm so glad I don't live in England, where there really is no choice! One of my friends has a ds 5 months younger than dd1, who started school a whole year before she did!!