No state school takes children into reception when they are 3. Children have to be at least 4 and in most areas now they start in the September following their 4th birthday, so they will be somewhere between 4 years and one day (!) if they have a late Aug birthday and 4 years 11 months+ if they have a Sep birthday.
Just to confuse matters, some areas still do 2 or 3 intakes a year, starting older children in Sep and the rest in Jan and/or Easter, so all children are then at least 4 and a half when they start.
Like someone said, some state schools have nurseries attached to them for 3 years old, but these are not compulsory, they are similar to playgroups and other pre-schools. However, if your school does have one, it is often to your advantage that your child goes to that nursery, sometimes they get priority for school places for example.
I think most local authorities require you to apply for a reception place in the October or November before the September following their 4th birthday. So in your case you would have to apply in autumn 2009. Letters offering the places then go out the following February, so February 2010 for you.
Like others have said, reception is not compulsory, but your child would then go straight into Year One, which could be harder to adjust to.
Schools vary as to when children go full time. At our school, the older children go mornings-only for a fortnight, then full time. Those born later in the year, say after Easter, usually go mornings-only until December, then full-time. However, other schools in the same town vary dramatically, some only do full-time, some do complicated arrangements of afternoons-only, then mornings-only, then mornings+lunch, then full-time! Each school really does decide for itself!