This is a very grey area and the employer is on very weak grounds. If the reason for the uniform can be 'justified' in terms of the personal or food safety reason, then that is fine. For example, it would be unthinkable that food manufacturers could not have a uniform.
If it is just for corporate branding purposes, it's dodgy.
If the employer dimissed, it would have to be for misconduct, and the employer, if he/she had more than 12 months continuous service might challenge it at an ET for unfair dismissal.