We always wanted to, but couldn't afford it from Day 1, so DS went to the local Ofsted outstanding Primary.
During the course of Year 2, our financial plans came good and we looked around 2 of the local Preps, falling in love with one of them. He had a taster day, loved it, so we registered him to start at Year 3. (Not London, so no 30 children for every place thank goodness).
For a few months, DS kept in touch with some of his old friends, but as many of their parents had been a bit
at our going Independent, that soon stopped (as indeed it did with our adult friendships). DS's friends are all from his current school, or from the several sports he does out of school, or from our village. All walks of life / schools.
It took DS a few weeks to break into existing friendship groups, but he has loved it from Day 1 and now he is in his final year of Prep with a position of responsibility, school colours in lots of sports, doing brilliantly academically and has lots of friends.
I know several people who've gone state to private and managed to keep their "mum" friends, but I only managed it with one, and she now lives in USA. Many of my old friends did seem to take the rejection of the Primary school rather personally, even though I never criticised it, only saying that DH had been to private school, adored it and wanted the same for our DS.