The 1851 Census lists her birthplace as Kelling, Norfolk.
"England and Wales Census, 1851," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:SGDJ-CRX : 24 July 2016), Sarah A Copeman in household of Richard Copeman, Cley Next The Sea, Norfolk, England; citing Cley Next The Sea, Norfolk, England, p. 16, from "1851 England, Scotland and Wales census," database and images, findmypast (http://www.findmypast.com : n.d.); citing PRO HO 107, The National Archives of the UK, Kew, Surrey.
Freereg has a record for Sarah Anne Jarvy born 20 Feb 1800, chr 9 March 1800 to Anne Jarvy in Kelling. (https://www.freereg.org.uk/search_records/5818dd03e93790eca3f9ba8c?search_id=59247b1f4325a63f59eb9e70&ucf=false)
Perhaps adding the Girdlestone to her name was a way of publicly naming the father, or shaming the Girdlestone family is another option! It's also possible she added it so as not to seem illegitimate at marriage, if indeed she was, I've had this happen before.
Family Search has the same Chr record, they list it as Hunworth, which is less than 6.5 miles South of Cley, but is about the same distance from Kelling. "England Births and Christenings, 1538-1975," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:J95W-1FJ : 30 December 2014), Sarah Anne Jary, 09 Mar 1800; citing Hunworth, Norfolk, England, reference item 2; FHL microfilm 1,526,778.
Of course it could be neither of these records or theories!