Dorset, derving its name from its county town Dorchester, is a country in South England bordering the English Channel. Stevens sends the afternoon of Day 2 on his journey here, and finds himself "getting lost down narrow, twisting lanes" while on his way to Mortimer's Pond.
Historically, considering its proximity to the English Channel and the rest of Europe, Dorset has had rather significant involvements in both World Wars. The first British unit to have to face a gas attack was the Dorsetshire unit. Only one village in Dorset had no deaths from World War I. World War II utilized Dorset's location to the fullest, and the British used Dorset heavily for preparations for the invasion of Normandy. Given the book is set after the wars have passed, but has flashbacks from before World War II, Dorset's inclusion given it's relevance in the war aids well to the looming war stories behind the journey.