How can anyone argue that forcing parents to spend money on a set of clothes the child can only wear at school is cost-effective?
While uniforms may be cheaper than non-uniform outfits, they are also an entirely unnecessary and additional expense. Families don't buy uniforms instead of street clothes (because the child needs something to wear when it is isn't at school), they buy them as well as street clothes.
In this respect, no matter how cheap uniforms are, they are still an additional expense.
The Guardian reported that 95% of parents from low-income families had difficulty meeting school-related cost. The additional expense of uniforms was a huge component of these school-related costs.