I’m going to assume this is a contemporary costume, because it wasn’t behind glass. But the aesthetic with the stars and the antlers… <3
Museum of London



It’s a Georgian-era fancy dress of Diana (a popular fancy dress costume) with a Yaseman Hussein metal wig commissioned by Philip Treacy, who designed the hats for the exhibit. There’s a fripperiesandfobs post saying describing it as a reproduction, but the Vogue article it links to only calls the Pleasure Garden a recreation. Many of the dresses are restored extant garments, so this one might be as well.
















