Earlier post was spot on for time of year (Jan for application, approx March Apr for allocation), however, it is generally not liked by the powers that be that any child is not in full time education. So, if you rocked up in Mid Oct in a new area the local council would take application and allocate a place as soon as possible. There is an appeals process as well.
In case you are not aware, all children in the UK should start Reception the September following their 4th birthday. it can be tricky to get decent info on schools as OFSTED ratings are not the be all and end all. i.e. There are 3 schools in catchment for me, all are GOOD according to OFSTED but every one is better than another in some respect. Always remember, you can always move school it is not a lifetime decision. Try to find info on a schools VALUE ADDED if you can