Good luck with that; unfortunately some of them prefer to purchase crisps, Haribo etc.... by that age, they can be starting to have influence over their meal choices and can suggest picking up half a dozen bananas or some oranges in the shopping instead of dairy milk based on personal preferences and their knowledge from school.
Jumpers are more affordable than Blazers.I'm not sure I agree. Blazers were the thing on my experience in 70s London and I don't recall it being any sort of issue.
Don't the majority of schools have a blazer policy anyway? And why a colour being a problem?
I would imagine a jumper would wear out quicker than a cheap blazer.
I don't see a lot of support for Blazers, except of course from headteachers or CEOs of Trusts, who want to deter poorer families choosing that school.
If you just stipulated a colour, and let people pick any jumper or blazer of that colour, you'd get different kids having different brands, which causes issues. Having a specific uniform acts as a social leveller, and reduces bullying.