I've just bought several jumpers, polo shirts and pants for my children online at M&S for £1 each.
I'm not sure if it's available instore.
They have quite a decent selection at the mo.
There is also a code for next day delivery - BTSNDDO8 - if you spend £15 or more on schoolwear.
Don't forget quidco too
My kids are relatively fussy so if they don't like them I'll take them back to a store for a refund ( the clothes not the kids!)