My top tip for this (I never spend more than £20 on school uniform each year), is to get organised and collect the size up when you spot a good deal on school uniform. It works well as well when kids' growth spurts happen in the middle of the school year, rather than during the summer holidays. 

Buy it at school 

Lots of schools now collect and sell 2nd hand uniform, raising money for the PTA at the same time, what's not to love?! This is a great option to source affordable uniform, especially if the school doesn't allow generic uniform, and insists that the clothes need to be branded. 

Facebook Marketplace / local charity Shops / Olio / Etsy

As long as things are in good nick (which I've found they generally are, since the kids grow out of them so fast), then have a hunt in these places for 2nd hand clothes, as well as shoes. 

Shops

If you can't find some pieces of uniform 2nd hand then keep an eye on deals no uniform at Aldi, Tesco,  Asda, Morrisons, John Lewis, and Matalan.