In terms of the ages, the only real outlier is the 1871 census (age 60) which puts her birth at about 1810 or 1811.
The 1841 census (age 35-39, rounded down to the nearest five years) puts her birth between 1801 and 1806.
The 1851 census (age 47) puts her birth in 1803 or 1804.
Her 1877 age at death (74 years) puts her birth in 1802 or 1803.
So we're most likely looking for a baptism sometime between 1802 and 1804.
The discrepancy in the place of birth could be an indication that she wasn't sure where she was born, which can happen when families moved around. If this is the case, then both Guildford and Stoke are likely places where she lived as a child. I would place less weight on the 1841 census, as it's much easier to get one letter wrong ("Y" instead of "N") than a whole place name.
So I searched for a Pratt family that lived in (or near) both Stoke and Guildford in the first decade of the 1800s. I found the following series of baptisms to a Thomas and Elizabeth Pratt:
If this is the correct family, then Fanny would most likely be the child baptised on 4 Jul 1802 at Stoke Poges. The record on Findmypast is from the Bishop's Transcript. Perhaps the parish register includes the first name, but these records to not appear to be available online.
Another piece of evidence that this may be Fanny's family is found in the 1851 census. Fanny Dowding is listed with her children at schedule no. 92 for Acton. On the very next page, at schedule no. 94, Sarah Pratt aged 46, a servant, born in Cookham, is listed. Thus, if the above hypothesis is correct, sisters Fanny and Sarah are living in close proximity to one another.
You might consider ordering Fanny's death certificate. You may get lucky and find that Sarah Pratt, or some other member of the extended Pratt family, was the informant.
