Yes Portsea is in the south of England. It could be that Frances decided to see the highlife and moved to London.
It did cross my mind that as the births of the children are on line, as is Benjamin's baptism and that of his siblings (and his parents marriage too), it is odd that his and Frances marriage doesn't show up. I wonder if they did actually marry.
In case you haven't got it, the parents marriage:
William Cowdroy to Sarah Powell, 6 April 1774, St.Oswald, Chester, Cheshire
Benjamin had the following siblings:
John James bap. 29.8.1776
Thomas bap.16.3.1779
Marie Elisabeth bap.21.3.1782
Caroline bap.6.1.1791
There also seems to be another child a Mary Powell Cordroy bap. 16.12.1813, but with a different mother, called Martha. The address then was Stalybridge, Cheshire, so this might be a different William Cowdroy.
Lizzie