Uniforms are funny things, if I'd have worn my Royal Navy Uniform as 'fancy-dress' (costume party), Id have been on a charge 'for bringing disrespect' to Her Majesty's Uniform, been fined money and given extra duties (cleaning/polishing mostly)....
ergo, wearing any 'Official' uniform to such a party is deemed to be showing disrespect to that particular uniform.

it's good that this uniform is only fit for costume parties, as for 'arry, he's just a spolit oik

now, your average party goer would be wearing an obvious homemade/rented 'mock'up', but borrowing one from P.Mike's of Kent was a no-no, unless it was covered in vomit of course....