This is a fantastic portrait of Miss Rushton from Lewisham, She is a Red Cross volunteer at London Bridge ARP shelter.
She appears to be wearing a man's battledress (it looks very large on her frame) and trousers. Her BD features an area making to left breast pocket with another smaller badge on the pocket flap (probably a red cross badge). She wears the ARP badge in her right breast pocket. The lanyard will be white as this is the colour worn in London.
Her helmet probably features the letters MU for Mobile Unit as pictures that appear with this photo show other nurses belonging to a mobile first aid post wearing helmets with these letters. The helmet also has a battery powered helmet lamp - an item of equipment rarely seen worn in WW2 photos. The battery pack in fixed on her waist.