Hi
Unless she was under age on her marriage (the age of consent was 21) she would need to be at least 21 to not need parental consent (which if she did may be specified in the church banns book, if it has survived, if the marriage was by banns or on the licence).
Have you found her still alive on any censuses? Have you checked the marriage register for any other Goater marriages?
The marriage according to the IGI occurred in Newington Southwark not Lambeth
MARY ANN GOATER 
RICHARD JAQUES  
Marriage:  16 JUN 1816   Saint Mary, Newington, Surrey 
Marriage coverage of the St Mary Newington registers on the IGI is 1813-1837. Coverage of baptisms is for the same time frame.
The IGI has this marriage
BENJAMIN GOATER  
DINAH SIMMS 
Marriage:  31 MAY 1785   Alverstoke, Hampshire 
This baptism would seem to indicate the couple moved from Hampshire
Diana Goater 
Christening:  05 FEB 1794   St Mary, Lambeth, Surrey
Father:  Benjamin Goater  
Mother:  Diana  
St Mary's Lambeth has very limited coverage on the IGI.
Regards
Valda