No one should ever feel they have to justify sending their DC to an independent school.
Unless they are all living of nothing but porridge.
I met a really lovely MNetter once, we all knew the others were MNetters, and the one who's DC was in an nonindependent school (rather than an Ofsted outstanding village school/Catholic school that only very special Catholics can get their DC into) felt the need to justify why she sent her DC to that school, which made me a bit 
There are no independent schools near me I would want my DC to go to, even if I could afford it for 3 DC. Which makes me happy in a way, because if I could afford it, I would either have to move house, or put them in a crappy school school which isn't as good as their state school because I felt I should.
I do sometimes stress that if my DC went to an independent school they would get better exam results at A'level than they would at a state school.
I go over, and over that one in my head. I, and my siblings got quite shit results at an independent school. (which got/gets decent results) I don't think we would have done any worse at a state school. 
The worse case scenario for me would be if we could just afford independent schools and the DC came out with rubbish GCSE/A'level results. That would be a killer.
So from education system I expect decent exam results, based on the fact I am offering the system little people with decent ability.
So far the system and recognised my DCs strengths and weaknesses and are working on them (mostly) to my satisfaction.