Wendy, if you google George Dodgson Callow, you can see examples of his paintings (Bonhams).

It's definitely likely that Benjamin spent his later years in London. His brother George D Callow married Katherine M Elston in 1866 and was living in London in 1871:

1871
London Fulham district 23, page 7
5 Park Terrace
Geo D Callow head 42 artist b Lancashire
Kate M Callow wife 31 b Northamptonshire
Bernard M Elston brother in law 11 b Northamptonshire

England & Wales, FreeBMD Marriage Index: 1837-1915
about George D Callow
Name: George D Callow
Year of Registration: 1866
Quarter of Registration: Oct-Nov-Dec
District: Northampton (1837-1923)
County: Northamptonshire
Volume: 3b
Page: 117

England & Wales, FreeBMD Death Index: 1837-1915
about George Dodgeson Callow
Name: George Dodgeson Callow
Estimated Birth Year: abt 1828
Year of Registration: 1875
Quarter of Registration: Apr-May-Jun
Age at Death: 47
District: Isle of Wight
County: Hampshire, Isle of Wight
Volume: 2b
Page: 366

The remaining task is to identify the Manx John Callow who was the father of the 2 Callow artist brothers.