It is for the governing body of a school (or in the case of academies, the academy trust) to decide whether there should be a school uniform, what it will be and how it should be sourced. To support them to do this the department issues best practice guidance which can be found here: https://www.gov.uk/government/publications/school-uniform.
This makes clear that we expect schools to ensure uniform costs are reasonable. Our guidance clearly states that uniform items should be easily available for parents to purchase and schools should keep compulsory branded items to a minimum. It also states that schools should avoid single-supplier contracts, but where schools do choose to enter into such contracts, they should be subject to a regular competitive tendering process.
No school uniform should be so expensive as to leave pupils or their families feeling unable to apply to a school and this Government has announced its plan to put the current guidance on a statutory footing at the earliest opportunity.