Vincini, I have been fetched from another thread to share my experience!
My daughter's case is almost exactly the same as yours - she was born 8 weeks premature in July instead of September and spent 5 weeks in scubu. I find her much younger than (some of) her peers, although she has no problems as such. She is also bilingual, which means that her language has up to recently been behind that of her peers.
She will be starting reception in September at the age of 5.2 and does not have to go straight into y1 - this is not impossible, as you have been told, but is harder to arrange in some places than others.
In our case, I bumped into a visiting educational psychologist at a surestart group when she was 3 and was lamenting the fact that I would have liked her to delay her school entry by a year, but that I had been told that this is impossible, to which he said that 'little is impossible for articulate parents who are prepared to demand what they want'(!) He gave me to name of the person at the LEA to write to and I stated my case and requested an assessment by an educational pyschologist. This happened and he recommended that she be back-yeared, so that she officially starts her schooling a year later. She is also going part time for the first year - I have discussed this with the HT. This means that she will not be in full-time schooling until she is 6!
By the way, it is not quite true that 'You are also entitled to request that she attends part time but the school does not have to agree to that request'; schools, by law, have to offer you part-time education until a child is 5, although many prefer not to let you know this. I am a primary teacher - I know this to be true!
I think what helped me was that the head teacher at her local school decided that she would support me - I went to see her because I did not want to go behind her back and possibly antagonise her before we start, and that the ed psych was obviously sympathetic.
Sorry this was so long and I hope it helps. If you would like to know to whom I wrote at the LEA (her position, obviously, not her name!) I can find the file and look it up for you.
Don't give up! And don't be bullied by schools and people telling you 'just send her anyway because everyone does it' - you know your child and have a right to decide!