But it does seem a shame if institutional/practical concerns end up being why a policy that could be helpful to some of the younger children can't be put in place.
But the policy proposed is apparently based on parental assessment only, which is in contrast to that of other countries. The policy would allow little Hugo with an April birthday to be held back just so that he has a better chance of getting into a superselective later, but might force little James with an August birthday to be sent to school with kids up to 17 months older because his single parent can't afford to pay childcare.
I would object far less if parents could defer only with the assessment and support of nurseries/schools, as the latter can distinguish more easily between children who are within the norm and children who could really benefit from starting later. But even in this system the problem is that children change a lot between 3 and 6: they could end up holding back a child who a few months later matured a lot and could easily be in with their age peers.
In the most extreme circumstance the proposed policy will end up just moving the cutoff from September to April, although I agree it seems unlikely everyone eligible would try to defer. More likely it will have the most effect in the (few) areas of the country that have selective secondary schools (which includes London). In those areas people who have July/August born DC could effectively be forced to defer our children just because everyone else does - who would want to put a child in a class where loads of children were 12+ months older?
Kent does have superselectives e.g Tonbridge which cream off the children who score highest in the Kent test.
Even leaving aside the issue of selective secondaries, some parents might hold back just to try to ensure their child was "top" academically: already somebody on this thread said this (mistakenly believing that oldest necessarily equals smartest/top). Given the amount of playground competition about reading levels etc by some parents, I wouldn't be surprised if a significant number of parents don't opt for deferring on these grounds.