@Chillisauceboss actually I know a few people who had September birthdays and were moved up into the next year, so I think not being stretched in your year group because you're the oldest in the year can be a concern.
Would you not be making applications in January, when your daughter is 3 and 4.5 months? My eldest is a late April birthday, so was 3 and 8.5 months when we were making applications, and although he is a shy child, I felt with my own assessment and input from his nursery that he was ready for school.
I understand the points about later life - although I have to say that, as a late August born child in a rural area it was quite nice that everyone else could drive before me and gave me lifts back from the pub I wasn't legally allowed to drink in.
All that said, it's entirely your right to request deferral and I understand your reasoning. I am not up on the current law and process, but is it still the case that it's the school's/LEA's decision as to whether your child gets to start in reception, or whether they would go straight into Year 1 after deferral? If so, and they decided your child should go straight into Year 1, would you then decide not to defer?
Finally, found this interesting article, although I haven't read the study it reports:
www.bbc.co.uk/news/education-44155068