Hi John,
This is purely wild speculation, but some things you might like to follow up.
I have noticed that prior to this baptism, there is a birth and death in Tasmania for a child by the same name with the same parents' names who died from influenza aged 4 months.
http://stors.tas.gov.au/RGD33-1-3-p048j2khttp://stors.tas.gov.au/RGD35-1-2p166j2kThis couple, John McKAY and Ann BAGLEY were both convicts and you can see their permission to marry, marriage and full convict records on the Tas Archives website.
http://linctas.ent.sirsidynix.net.au/client/en_AU/names/?
Ann's conditional pardon was approved in November 1847 and her term of transportation expired in 1850 (one extra year for larceny in Tas.) There were two John McKAYs, but this one's marriage documents show that he was the convict on the Elphinstone 2 (second voyage) which arrived in 1837. His description document also confirms that he was a coachman and groom which matches his daughter's birth and death records. John's term of transportation expired in 1843.
Going to Georgina's marriage cert, the three witnesses are William GARROD/GARRAD - he was living at 4 Rheidol Terrace in 1871 (same address as Charles when married) and was also a carpenter so it is likely they worked together. Charles is not there in 1871. Sarah Ann WARD was living with her parents William and Betsey at 18 Rheidol Terrace in 1871 so Georgina may have moved there prior to marriage.
The third witness is Ann HENWOOD and I believe she is the same person who in 1871 is living with her husband Arthur in Islington. There is a marriage in 1864 for Arthur HENWOOD and Ann McKAY, both widows. The census records her birth details as c1817 at Wisbech in Cambridgeshire, which is similar to convict Ann BAGLEY who was tried at the Old Bailey in London but was a native of Cambridge, aged 24 in 1840.
When this Ann married Arthur HENWOOD she said her father was 'William Lewis' a Millwright, and you would generally assume that LEWIS was his surname. However there was a William Lewis BAGLEY, Millwright, who was having children baptised in Islington in the early 1800s. His son William Lewis was baptised twice, firstly in 1821 when he was born, and again in 1828 (strangely by the same clergy) with his sisters Ann (born 1816) and Mary Ann (born 1819). I don't know where the children were born. William Snr. had died in 1820 aged 40 just 2 months before William Jnr's birth.
I also noticed that there is a baptism for an Ann BAGLEY, parents William and Mary at Wisbech in 1815. Might be interesting to see the occupation on that one and check the year - I don't have access to an image.
The London William and Mary were married at Lambeth:
https://familysearch.org/ark:/61903/1:1:N66R-SBWDebra