Starting at the end, discharge took place in UK but knowing how long he spent in hospital overseas and how long at home I would not like to guess, much would depend on the severity of the wounding. There are all sorts of scenarios, each probably as valid as the rest:
Wounded at Gallipoli, evacuated by hospital ship to Egypt while Gallipoli still going on, evacuated to England, discharge.
Wounded on the Suez Canal (there were minor engagements before Romani, there didn't have to be a named battle going on for folk to be wounded), hospital there and later back home.
The length of time between the end of the Gallipoli campaign and his discharge suggests either severe wounding there that took a long time to treat or wounding after Gallipoli at who knows where/when.
Not really a best guess, just some ramblings! Proof of any scenario- unlikely, I lean to the after Gallipoli scenario.
MaxD