I was struck by the way the film ended. I didn't start thinking about the book Journey's End until I saw "I" pack up and basically leave Whitnail behind. It could have easily been a situation where they both are still living together, or both get on the train together and leave, but they don't. "I" is leaving Whitnail behind and he is not returning. Whitnail is left holding a bottle of liquor in a rainstorm as he walks back to a flat he is being evicted from.
I was really taken by surprise the sadness of this ending especially considering it was a comedy. I've seen this happen to friends in real life, where one moves on and the other is left behind, there is a sense of realness to the end of Whitnail and I and the film would be less if not for the sad ending. As
@Zer pointed out, Whitnail was talented, more talented than his friend, but for whatever reason his fate was much darker than his friends and he was left to it. I honestly thought the film was just o.k. until the end. The ending makes the film something else entirely, it elevates a nonsensical comedy to a study on life and friendship.