My company has changed our uniform to something hideous. Do I have to wear it?

If you are contractually obliged to wear a uniform, you would be unlikely to be able to claim a breach of contract on your employer's part, based solely on cosmetic changes to that uniform, however distasteful.

If there is a union in your workplace, and the changes are broadly unpopular with the staff, you could take up the issue with your union representative, who would be able to negotiate the changes on your behalf.