Precisely that, yes. Britain wanted to make the Canadian colonies safe, and so establishing a secure overland route for communications by taking Aroostook and some border territory was seen as paramount in importance. I learned that Palmerston himself detested the territorial changes in the Webster-Ashburton Treaty, and thought they should be changed. I figured he would push for that rather than land on the Pacific that he, and to my knowledge few in his cabinet, showed any interest in. They might kick themselves for it down the line, but it was not the seat of the war, nor forefront in their minds. I did toy with giving more territory to Canada, but I figured that was not what Britain wanted. Navigation around the Pig Islands (San Juan Islands for those who want the exact geography) was important, but expanding British Columbia was not.
A buffer for connecting Canada to the sea, yes, so much territory to garrison and police while potentially making the United States as a whole mad, no. The people of the region who are die hard patriots will be gone by 1866, while those who shrug and get on with life will still be there. T
Prince Edward Island will have competition for who has the most potatoes post-Confederation...