Hi Carly,
It sounds like Frederick and Elizabeth Anne / Annie were for whatever reason unable or didn't manage to marry in England. You mention a "first marriage in England" so perhaps they were unable to legally marry.
I can't find "Frederick Gately Townsend" in England. There is a Frederick Townsend born in about 1840 in Cornwall, who was married to an Elizabeth by the 1871 census.
https://www.britishnewspaperarchive.co.uk/ has some references for a Frederick that might be of interest
Published: Tuesday 12 October 1880
Newspaper: Western Morning News
- bankruptcy of Frederick Townsend, draper, grocer, general dealer
Published: Saturday 06 November 1880
Newspaper: Cornish & Devon Post
- Elizabeth Townsend takes out a protection order against Frederick her husband, who had left her without cause.
https://www.familysearch.org/ark:/61903/1:1:Q27X-WPZC - Elizabeth living "married" in Cornwall.
https://www.familysearch.org/ark:/61903/1:1:WH73-NW2So probably Frederick took the name Stuart to avoid his creditors and his wife back in England. Perhaps he ran off with Annie Turner, or met her in Sydney?
I would look at -
The birth record for brother Frederick, 1888 - any indication of "previous children of marriage"?
The 1908 marriage - what are the details of Annie / Elizabeth Annie?
The death of Frederick - who gave the information? Were all of Frank, Maud, and Frederick left off? Very unusual in a case like this to report the children of the first marriage...
See if you can put together a timeline of when Frederick got to Sydney and what Annie/Elizabeth Turner was doing in her earlier years - when did they meet? Perhaps Frank was from a previous relationship of Annies?