Rather puzzling.
Judy says her uncle was born in 1901 and served in WWI. He looks about 40 here and would, I think, have been called up at the outbreak of WWII. In which case, as has been previously said, why is the junior rating wearing a cap tally for "HMS Centurion." During the war only "HMS" cap tallies were worn. Jolly Jack should certainly be getting a bollocking for wearing his cap flat-a-back.
HMS Centurion was built as a Thunderer Class warship in 1912/13 and was converted to look like a King George V Class Battleship, to confuse the Germans, in 1941 for blockade duties at Tripoli. This URL gives details
http://www.bbc.co.uk/ww2peopleswar/stories/21/a6354821.shtml . The name HMS Centurion was from 1956 applied to the shorebase at Haslemere of Commodore Naval Drafting and from 1970 to the Drafting and Pay section in Gosport.
Judy's Gt Uncle was certainly a PO and his branch badge (right arm) looks like that of a Naval Airman although it is too blurred to be absolutely sure.
This URL may be useful in searching for her Gt Uncles records
http://www.nationalarchives.gov.uk/news/stories/78.htm