The first confusion to clear up is that the numbers 7+, 8+, 11+ and 13+ describe a child’s age at entry to a school, rather than the age at which the family first applies. This distinction matters as for some 13+ schools, the admissions process begins while a child is still in primary school.
Much of the confusion stems from the fact there is no single application timetable for British independent schools. Each school chooses its own registration deadline, assessments and offer process. So, the most useful starting point is to decide which type of school might suit your child, then work backwards from the requirements of the schools you are considering.