First of all, i have to clarify that not all private are selective.
My wife is a secondary school teacher. Based on her experience at several schools, she said no discussion of how to help high attainers among teachers meeting. All they talked about were how to keep the reluctant and the lazy ones at school, how to keep them happy by allowing them to cherry pick which lessons to attend, some trouble makers may only come in the afternoon and leave at any time as they please.
Like it or not, there are serious issues in our state comprehensive system. Pupils are rewarded for bad behaviours, good students are being neglected, teachers are put off by the workload and no punishment for bad students.
There are many canadian teachers teaching in UK schools, many quit uk for other countries long before their contract expires. They said the schools here are terrible! Long hours teaching, too many classes in a day, too many students in a class, teachers are like beggers pleading students to learn, too much time spent on managing the bad students in class, no real punishment for laziness, too much protection for students, etc etc.
So if we can afford, we will 100% choose private. Comprehensive is not an option to us at all.