My daughter goes to Old Palace of John Whitgift, Prep. The school is 3 to 18 and its not seen as a heavy hitter academic school, it's in the Times top 30 for London indie schools, and it does better than most state and at least as good as many grammars, but it's not known as a hot house or anything.
My DD loves it, and joined in nursery. She learns languages, they have their own swimming pool, large playing fields (over in the prep) and has beautiful Elizabethan buildings in the senior. She does clubs from Yoga to tap dancing, after school clubs, breakfast clubs, there's always trips to plays, museums and concerts, a well stocked library. There's a really good community at the school. I haven't met a bad parent, and the teaching is first class. There's a real mix, while some are very wealthy and live in huge houses, there are many who aren't and live modestly and are just making big sacrifices to send their girls there.
So although my DD isn't at what I would call a very academic prep, she's receiving a brilliant education, regardless if she stays on after y7 it has given her an excellent start in life, which is what we wanted.
If you're going to really struggle financially, then that's a different question. I pay nearly 13 grand a year in just fees (and that's prep, it jumps up at senior school after Y7 to 17 grand, ) there's always lots of extras on top of the fees- uniform replacements, trips, clubs, birthday party presents
So the fees are just the bare minimum.
For me it's precisely the soft skills preps are really good at- communication, team work, self confidence, creativity especially. I went to state primary and couldn't believe the music opportunities my DD has at Prep, better than my state secondary.