Different strokes. It depends on where you see the value add and if it is worthwhile. I definitely agree you can do all the 11+ yourself on top of a state primary education. I will never disagree with you on that. My point is that it's reductive to put it simply in terms of Maths and English.
We've come out the end of this experience (well... the end is tomorrow hopefully but still...) thinking that we probably could have done the academics side ourselves at state, it would definitely have been cheaper!
But the support we've received from the school, even the simple fact that all of DS's friends are going through the journey together is a huge factor. The biggest thing though is DS's experience, within two terms he loved the school. His descriptions were that his previous school was always "boring" and now, even though the prep is "still school", there is always something to look forward to. He's massively developed his confidence, now performs upwards of 10 times a year in front of a public audience through the school and values learning more than he did before.
Is that worth £14k a year and the risk of him becoming a self-entitled brat? Everyone will have their own take, for us right now it is a yes, but ask me again later as I'll probably change my mind on it every 10 years.