DC1 is now 12 and I was looking through his memory box and found the class list for his nursery (it was at a school foundation going from 3-18).
Of the 16 children:
1 left before the end of nursery year because of illness.
1 left in Reception as she got into a good local state school.
1 moved to another indie school nearer her home in y1. Another moved abroad.
in y2 1 left to a high-powered academic school. 2 moved to good local state schools, 1 moved to an indie faith school, 1 moved to a different indie.
in Y7 1 moved to a state grammar and 1 moved to a single sex school.
So of the 16 children who started nursery, just five are still in the foundation.
Your choice of school at age 3 is important, but children do move. We all make the best decisions we can at the time with the information we have. Hope this helps ease some of the pressure.