I know I'm a bit late to this chat, I deferred my own December born child due to health issues and lengthy hospital stays during the 2 previous years, and we were granted additional funding to stay at nursery. August to December born children are applying for additional funding really as you do not need to be in school until you reach age 5, and we only have 1 intake. In years gone by there were 2 intakes per year (showing my own age there).
The legislation is very clear, if you are 5 you must start school unless there are exceptional circumstances, and unfortunately GDD isn't classified as exceptional, and schools should be well set up to cope with additional needs (inadequate funding is a major issue though). The system needs cut off points in place otherwise it would be a total free for all and a class could be made up of 4 -7 year olds, how would a teacher cope with that range of ability in a class, impossible I would say.