Most independent schools have a uniform – sometimes it will not have changed for a long time (plus-fours, tail coats). It will usually mean buying a special tie and blazer, skirts/trousers, shirts, shoes, jumpers in the right colour and special sports clothes and equipment. Many schools seem to have endless lists and you could also be required to buy school colours for sports teams, summer clothes, formal dress (maybe a kilt for a Scottish school), coats, macs, smocks for art, etc. Schools will let you know where to buy the uniform, including any second-hand sources, before your child joins. Don’t make life hard for your children by sending them to school with the wrong uniform.
Many boarding schools allow pupils to wear clothes of their own choice in the evenings and at weekends. In the sixth form, uniform may be subtly redesigned to distinguish its wearers from the lower forms, or quite often it is waived completely or replaced by a dress code.