Kay is cosplaying as Saber/Artoria Pendragon in her "British Travelogue" outfit, which was featured in a 1/7 scale figure. Here are some references for the figure:
https://www.1999.co.jp/eng/10511478; https://myfigurecollection.net/item/645869
(I was able to get one direct from the Aniplex booth at Anime Expo a couple of years ago. Saber in that outfit was too pretty to pass up!)