They were rehearsing for a Christmas Charity it seems.
Putting on Drag Performances were apparently common in the british armed forces to boost morale.
https://www.forces.net/heritage/history/did-ww2-soldiers-fight-hitlers-nazi-germany-while-drag and https://rarehistoricalphotos.com/british-soldiers-manning-anti-aircraft-guns-in-womens-clothes-1940/
IDK why that Context isn't in the Title, but here it is.