Address for 1871 Census should be Platte Saline

. I'm assuming that you are certain of her birthdate so if that is the case this appears to be her in the 1881 Census:
Name: Elizabeth Ball
Age: 29
Estimated Birth Year: abt 1852
Relation: Servant (Head) (Servant)
Gender: Female
Where born: Alderney, Guernsey, Channel Islands
Civil Parish: Kensington
County/Island: London
Country: England
Street address: 12 Sheffield Gdns
Education:
Occupation: Cook
Registration district: Kensington
Sub registration district: Kensington Town
ED, institution, or vessel: 9a
Household Members: Name Age
Elizabeth Ball 29
Maggie Lewis 24
My guess is that perhaps her father was on Alderney either in the military or to do with the construction of the breakwater at the time of her birth.
Regards
Jan Herivel