According to Abba: The Official Photo Book, published to mark 40 years since they won Eurovision with Waterloo, the band’s style was influenced in part by laws that allowed the cost of outfits to be deducted against tax – so long as the costumes were so outrageous they could not possibly be worn on the street.
That’s moving the goal posts and completely irrelevant; of course it’s case-by-case when it comes to what constitutes a ‘uniform’, or else no clothes would be considered non-deductible as anything could be a part of a uniform.