State schools should not be allowed to specify expensive uniform, then 'not being able to afford it' would be much rarer.
DS's primary school 'uniform' required white shirt/polo shirt/blouse, dark grey or black trousers/shorts/skirt and a blue sweatshirt/cardigan. Sweatshirts and polo shirts with the school logo on the front were available through the school, but not compulsory.
You could go into Asda and buy cheap versions, or into M&S and buy expensive ones. They all looked 'matching' and could be spotted easily if out and about, and nobody was under any pressure to wear anything expensive and fancy.
Even his little French classmate, who found the idea of wearing a uniform bizarre, was kitted out with a 'capsule wardrobe' which allowed her to pick her own outfit each day.
Everybody happy, no excessive cost. Easy.