Children can go to school the september AFTER they turn 4, if their birthday is before September 1st.
You will get a form from your local LEA, where you will be asked for your 'preferences'. Note, there is no guarantee that you will get into your chosen school, or any of your preferred schools. All schools will have a criteria list which should be available from the website. Church schools will be slightly different again, but, you still need to complete the form from the LEA and may have to provide secondary forms if its a church school.
The criteria are usually,
Children in 'care'
Siblings of children already at the school
(if it is a church school, then children of regular church goers will come next)
Distance.
These are just a guidline, please do check the school website, and your local LEA website.
but, it schools are NOT allowed to have a first come, first served system and even if your child has attend pre-school or nursery attached to a primary school this will not guarantee you a place there.
Children do not HAVE to attend school until the september after they turn 5. However, if you do choose to defer, your child will go straight into year one, without doing reception year. This has many drawbacks including, lots of basic education will be done in reception, number recognition, letters, writting, colours and so on, and perhaps as important, social bonding between the children will be started and so a deferred child will be the 'new' kid in year 1.
Of course, after all that, you could always choose to home educate, but thats a whole different kettle of fish.
Hope that helps a little.