The word "journey" is the most suitable option to complete the sentence. The sentence is talking about traveling from London to the north of Scotland, and a journey is a more appropriate term to describe this type of travel. A journey can refer to a long trip or travel, especially by land, which fits the context of taking an overnight train. The other options, such as flight, trip, travel, and voyage, do not accurately convey the idea of traveling by train over a long distance.