The government produced new statutory guidance on the cost of school uniforms in England, most of which applies fro. September 2022.
The guidance is here.
This is the summary that goes with it:
Parents should not have to think about the cost of a school uniform when choosing which school(s) to apply for. Therefore, schools need to ensure that their uniform is affordable.
In considering cost, schools will need to think about the total cost of school uniforms, taking into account all items of uniform or clothing parents will need to provide while their child is at the school.
Schools should keep the use of branded items to a minimum.
A school’s uniform policy should be published on the school’s website, be available for all parents, including parents of prospective pupils, and be easily understood.
Schools should ensure that their uniform supplier arrangements give the highest priority to cost and value for money (including the quality and durability of the garment).
Single supplier contracts should be avoided unless regular tendering competitions are run where more than one supplier can compete for the contract and where the best value for money is secured. This contract should be retendered at least every 5 years.
Schools should ensure that second-hand uniforms are available for parents to acquire. Information on second-hand uniforms should be clear for parents of current and prospective pupils and published on the school’s website.
Schools should engage with parents and pupils when they are developing their school uniform policy.
I would start by looking for the policy on the school's website and then ask how they have met the requirements.