My DS1 is in Y8 know and gets out of school at 6pm.
DS2 is in Y1 and gets out at 3.15, it increases each year.
School opens at 8am.
However, many of the private central London schools only have lessons up until noon on Fridays, including ours.
But, as someone said, traditionally kids in private schools would either have full time nannies, or mothers who aren't working (or both!), and many of the kids will enroll in the school's (pricey) holiday clubs, travel abroad and definitely continue to read and write during the holidays.
I love the long holidays, but it does feel a bit like the more you have the more you get. We get cheaper holidays (not that private schools generally has anything against taking the child out of school during term time, no fees or anything like that), we can attend museums ans parks when they are super quiet, go to lego land and similar with off peak prices, etc etc.
We get enough time to either travel or do a big project at home, which is nice. Our eldest is currently off skiiing in Austria (arranged by the school as a holiday club).