Firstly you should know you have the legal right to "request" that your dc starts in Reception a year after she should, but that is all. You have no legal right to start her in Reception the year after she should.
You have the right to not send her to school, but the authority does not have to 'reserve' an out of year place for her, and starting directly in to Year one would be extremely challenging, IMO.
Secondly, she will change massively during this year. She has about 1/4 of her life again to continue to grow and develop during this year.
You also have to think about what happens when she is older....... what about when she should go to secondary ? At the moment, unless it is written in to an EHCP, there is no "right" to start secondary a year late either - can't imaging a jump from Yr5 to Yr 7 will be helpful.
Is she in an area where they take tests to 'pass' to their new school? How does that work?
What about if she wants to play in sports teams, and the governing bodies (quite rightly) only let people in the right age brackets play - she will be outside of the Teams all her friends play in.
I (notr anyone else on this thread) can't tell you what to do, but you need to go in to it with your eyes wide open. This isn't just about your current 3 year old, you have to think about when she is 11 or 14 or 18 too.
IMO, Reception is the MOST accommodating year for being aware of what can be considerable maturity differences between pupils.