I did this with ds2. He is a sept baby and had been in his ind nursery since he was 6 mths, so i felt a change would be a benefit. he is also very quiet in nursery ( he only started speaking at his ind nursery at three and a half), finally none of the children at his nursery would go to his school, and I felt mixing with children who would go to his school would be good.
He started in Sept and I have to say it is the best decision we made. He has settled well, the staff ( and school staff) have got to know him and he is starting to make friends with those who will be in his class. I work 4 days a week, so a cm picks him up and drops him off, but this would have to happen when he started school anyway, so he has also got to know her.
We were lucky that ds1 already went to the cm, so we had that set up.
On the flip side ds1 was at his private nursery until he went to school, and only knew one other child, and settled well.
On the learning side, the pre school does 'letter of the week' which his nursery didint do, but otherwise is the same. However, it does prepare children in the way the attached school like IYSWIM, and , in hindsight, I think ds1 was at a bit of a disadvantage because he was slightly out of step witht he pre schoolers.