Hello and welcome! I agree with what Nick has said, and here are some additional tips.
Though your child is eligible to start school in Reception in September 2013, she doesn't have to. She reaches compulsory education age in the term after her fifth birthday, so September 2014.
Local Authorities in England tend to be exceptionally keen on keeping children with their "age peers", so the cutoff dates are strict and it is quite rare for a child to be placed with older or younger children. This means that if you wait a year for your daughter to start school, she will go into Year One, skipping Reception, and will still be one of the youngest in her class. Some people seem to be of the opinion that missing Reception would be an academic and social disaster, but I personally think this is unduly alarmist.
If you feel your child is too young for full-time school at just-turned-four, you could send her to nursery (preschool) instead of Reception. Her eligibility for the funded half-time nursery place continues right up until she actually starts school (at which time the funding is diverted to the school instead) or until she reaches compulsory school age. The curriculum for four-year-olds is theoretically identical in both educational settings (school and preschool), but I think that preschools' expectations often can be more realistic and age-appropriate, and they have a much better adult-child ratio. And preschool is half-time, which also makes a difference.
In parts of Oxford city, the schools are bursting at the seams with young children. Cost-cutting exercises some years ago combined with a slight recent demographic shift have left some children without a suitable school place. For example, they might be sent to a school which is not near their home, or siblings can be allocated to different schools. If your child is unlucky enough to be in this position, you could put her on the waiting list for the school you prefer and hope someone will move away and free up a place. Oxford has a very mobile population, so that sometimes works. While you are waiting for the school you want, you could keep her home, send her to nursery or send her to whichever school you've been offered.
If for any reason you don't want to send your daughter to school just now, you might like to join in with home education activities. There are many people home educating in Oxford for all sorts of different reasons, so there is plenty to do! www.ohed.org.uk/