HerBeatitude I can see where you are coming from. But if you feel really strongly about this issue, then use the policies of the school to force the issue out into the open.
As a parent, you have the right to see any policy and most governing body and committee minutes.
Ask for the minutes of the meetings at which the uniform issue was discussed. (Do not let them fob you off with "theres confidential information in the minutes" because any clerk worth his/her salt will not have put anything personal in there. Personal stuff should be held seperately, and no child is ever named in minutes)
You also have the right to ask for, and get, information on the profit made by the school either via a 3rd party supplier, or direct from the school. There is clear guidance that schools should not be making a big profit from the sale of uniform.
You should ask for a copy of the schools "best value" policy (ie a policy on how it goes to tender, how it chooses suppliers) and ask how it applies to supply of uniform. (Ask when the supplier was last reviewed, who else was asked to bid for the role, and ask to see details if available. Ask what steps the GB are taking to provide competative choice for parents)
You could ask that the governing body survey the parents in the school to determine what parents feel is reasonable. You should speak to the parent governors and ask them to take it up in the next meeting.
ANd then ask to see the minutes.
Raise it as a formal complaint. Every school must have a complaints policy. The normal process is Head > Chair of governors > clerk to governing body (and therefore governing body) and then, if you feel (and can prove) that the school is being unreasonable, the ombudsman or the secretary of state for Education, Michael Gove.
You could also speak to the LEA and ask their advice on this, especially if you feel that the uniform policy is causing financial hardship for your family. Many LEAs have a hardship fund to help buy uniforms. If they get a number of requests from one school,you better believe they are going to have words with the school...