From 2011, all state schools in England will have to offer school places to children in the autumn after their fourth birthday. However, it is also clear that parents have the right to defer their child's school start until later in the Reception year if they prefer, without losing the school place. Parents also have the right to choose part-time school until their child reaches compulsory education age in the term after their fifth birthday.
vixcut, if you feel your children will be too young to start school just after they turn four, there is no rush. If you want some activities to join in with and other children for them to play with during the daytime, you could join a local home education group. There is plenty going on in Oxfordshire: www.ohed.org.uk/ Many home ed groups welcome families with preschool-aged children who want to talk through their options.
I don't agree that waiting to start school puts children at a disadvantage. That assumes that children won't learn and mature outside of the school environment. I also think people worry too much that their child will be left out socially if they start school later than their peers. Nobody worries about this at nursery, where children join throughout the year. If parents plan to move house, everyone reassures them that children are adaptable and will make friends. Particularly at four or five, children can be quite fickle about their friendships anyway: it isn't likely that they've made friends for life at this age!
My daughter started school considerably later than most children do in this country, without any previous formal education. Because she was old enough for it, and hadn't had her confidence dented by being asked to do things before she was ready to do them, she had no trouble academically. Far from excluding her, the other children were all curious about the "new girl" and eager to be friends with her. Neither she nor I had any anxiety about her school start, because the time was right for her. That would not have been the case when she was four.