All totally fair enough Elaine, but then surely the answer, from you, to OP would have been: "Haven't made this decision, but imo, never worth the sacrifice as I think its morally wrong to choose private because it perpetuates a two tier system, which I don't think is right". You probably did say this, I am just not trawling back! From you (totally understandable) viewpoint, it doesn't matter in the least whether prep schools or any other private schools are worth the money, as they are wrong!
I am, in this narrow way only, I hope, nowehere near as principled [and also v coloured by my own background and upbringing from which it would be seen by fam,ily etc as frankly odd if I chose state - not that that is really a big factor for me]: I recognise that its a two tier system and that that is totally unfair. However, i want my DCs to be in the top tier, I can afford private, and so, in the interest of doing what I think is best for them, will choose private.
OP, if you're still there, I would look at what you can afford for, say, the next 3 or 4 years. if you could, even if it were by economising, afford private, then bear a pre-prep/prep in mind. Look round the prep AND your local state schools, really think about it and what the differents costs, in the widest sense, would be for you, and choose the school which you think is best for your child now. I promise you that if you choose the choice that you really think is best, faced with what you know now (none of us have a crystal ball), you will not regret it.