Private primary round here is around 5k pa. Seniors are upwards of £7k. Some schools e.g. Girls Day School Trust have fees which remain static from Y7-Y13 at around £7k and there is a philosophy of continued education (read they will try to help you through a time of financial pressure so that your child's education is not interrupted).
Others have a rising scale of fees - day pupils in Y7 start at £9k and by Y11+ that is almost £11,5k. Boarding is an extra £2.5k approx. Repton is also local-ish. Fees there are around £15k pa for day pupils.
Extras for all schools are things like music lessons - around £150-£200/term.
Most parents I know buy uniform secondhand either from the school shop or from each other. At Foremarke/Repton they also sell sports equip in the secondhand shop esp equestrian, my friend tells me.
Parents IME are a mixture with some people having tons of money to splash around and others not.