<Snipped quote by MacabreFox>
Assuming she wants to pose as a Brit, something close to what she actually is works. For example, her disguise can be the representative of a women's volunteer nursing group. Alternatively, Evelyn can say she's the daughter of a rich family, and ended up talking for the family business because her brothers are busy soldiering. You can also combine these and/or some the suggestions below.
-Promoter for female factory workers
-Wealthy socialite
-Performer; she will be singing God Save the Queen at the beginning of the party
-Journalist of a small newspaper/magazine
Perfect, I like these ideas a lot.
So then, Evelyn's alias, and cover story, will be Emily Roberts, the Performer.