A cascade of poppies falls from ‘weeping windows’ across Britain. A 50-metre drawing of Wilfred Owen appears in the sand, and is washed away by the sea in which he swam. A silhouetted soldier stands on the white cliffs of Dover. A thousand pumpkins ‘recall’ an antisubmarine airship. You can pretend you are in no-man’s-land in Dorset, or ‘clearing up the immense horrors of trench warfare’ in Dulwich.