Few of us came from low income backgrounds. Many went to primary schools in affluent areas. Their parents paid for piano lessons and extra tuition. They were families who went skiing in the winter and bought branded food from the supermarket. Often these children were privately educated, went to posh prep schools where they are relentlessly trained to pass the 11-plus. Grammar school was a way for affluent parents to save money on educating their children. And it still is. It is largely a preserve of the middle classes.
My article on grammar schools.