We're north of England and made the state primary to private secondary move. We had the added extra of the end of Y6 coinciding with the first lockdown and his state school's provision was dire. I could see from social media that the kids in Y6 at the private school were getting really good lockdown provision, so we had that extra stress/worry.
I would say the biggest changes/differences he found have already been echoed by many others - sport, music, languages, as well as the general ethos of responsibility, homework and time management! We'd bought in to the local music service provision so he was actually 'ahead of the game' when it came to that, but the other aspects were a big step up/change. It wasn't just the amount of sport they played, but the type. At his primary it was lots of 'navigation games' or 'dance and movement' and no team sports at all; he's had to learn the rules of rugby, hockey and tennis pretty quickly! The move to secondary always comes with a jump for homework & time management, but those who moved up from the private junior school seemed much better prepared and we've had to work hard with him on that part.
Our daughter will make the same move next year and we've learned a lot about how to ensure she's bit better prepared. She's sporty, so we've got her involved in the local hockey club. Mainly so she knew the rules already, but its come with the added bonus that her team is primarily made up of girls who are already at the private junior school and will be in her year when she goes into year 7. We're also trying to work a bit more on the time management and responsibility side of things, so its not quite such a leap.
It's certainly worth finding out how many join in year 7. Many schools increase their numbers in year 7, so there will be others in the same boat. Ours takes about 30-40 extra kids into year 7 and they mix all the existing classes up to help the new kids integrate quicker.
From the money side - we've gone from relatively 'high' earners among the state primary parents (when discussing things like holidays our kids always seemed to be better travelled than their state primary peers), to 'lower end' at the private secondary (lots of lovely holidays and cars that are well out of our price range). But, the more parents I've met the more I've realise there really is a wide spectrum and there are plenty of us that inwardly groan every time we find our little Peugeot sandwiched between a Lamborghini and a Maserati on the morning drop off loop.
We actually gave our DS the choice - he could go to the local comp with his mates (not really our choice, but we wanted him to be happy) or choose the private secondary. He chose the private school and he's said many times that he doesn't regret the choice he made. He's got a wide group of friends, some of whom came from other state/private primaries and some who had already been at this private school since year 4. Two years on and I'm not sure they can remember who came from where now!