Cotswold,
I would say that there are two main arguments against.
One is that you may well have an extremely limited choice of schools. The school cannot 'hold over' a place for your child to start in Year 1, so unless the school is always undersubscribed, with space in Year 1 every year, you will just get whatever school has a place at that point ... in some areas that will NOT be in a good school and may well be a long distance away.. Even if the school turns out to be undersubscribed in Reception, EVERY applicant during the Reception year will leapfrog you - so if the class has e.g. 27 children in September, a few new arrivals can scupper your chance of a place totally. My DD's class accepted its first new pupil from the waiting list / in-year application at the beginning of Year 3, as there had been NO movement amongst the original 30 for the previous 3 years.
The other issue is that in most schools, the start of Year 1 means the transition from play-based to much more formal learning. If you think that your young-for-the-year child is not ready for the start of play-based learning in September, how much less ready will she be for formal learning a year later, when all of her classmates will have worked slowly up towards it and she will have to go straight into it?
There are a couple of circumstances in which the impact wouldn't be too bad. One would be in a small school with a mixed Reception / Year 1 class. Your child could join that class as a Year 1 but access the play-based Reception curriculum for as long as needed during that year, which would hopefully prepare her for the transition to Year 2 at the end of the year.
The other circumstance where the impact might well be less would be if she has access to a fairly full-on play-based education for the Reception year, just not in school. So if, for example, you actively home educate, or if she attends a pre-school or nursery willing to take her until the end of reception, or if she attends an 'alternative' educational setting, where she would get the progression in skills and knowledge that she would need to prepare her for the beginning of Year 1