"Claud, who had served in the Anglo-Nicaraguan wars of '46"

This comment gives us the best clue as to how far into the future the events of the story are set.

Stella Gibbons may have come up with the Anglo-Nicaraguan wars because Nicaragua (the largest nation in Central America) was going through a turbulent phase in its political  history at the time Cold Comfort Farm was written: the country had been under American occupation since 1912, and would gain its independence in 1933.