Uniform will vary. I worked at, and my daughter went to, a prep at the cheap end. Specialist uniform and kit included: blazer, jumpers, kilt/pinafore, summer dress, shirts, PE top x2, skort for older years, hoodie. You could get away with supermarket polo shirt (younger years), PE shorts, joggers, tights, socks, shoes, trainers. They did have a good second hand shop for the branded bits to be fair.
I know of an another local school which specified a particular shade of maroon tights right up til year 11 which were not easily obtainable outside the special shop.
Wrap around at mine was actually cheaper than the local state school and included a proper tea, not just a slice of toast.
Music and drama lessons are optional and also probably no more expensive than at the local state.
School trips are likely to be more frequent, more expensive and no option to opt out of the voluntary donation if you’re having a difficult month financially- they’re just added to the invoice for next term.