Of course, recognition of Mr. Thornton's worth has been slowly growing on Margaret for some time, which is why she wanted Nicholas to ask him personally for a job. Unconsciously or not, she believed Thornton would not be as vengeful and unwilling to forgive as many of the other mill owners. This is why she is so disappointed when Higgins tells her that Thornton refused to give him work; earlier in their acquaintance she would not have been surprised- indeed, she would have expected it.
It is at this low point that the letter arrives from Edith saying that they are returning from Greece to London, and would love to have Margaret with them. At the beginning of the novel Margaret, care free and happy, couldn't wait to get away from the shallow pursuits of London and live in Helstone with her parents. Now, with Helstone a distant memory and her mother gone, Margaret, tired and sad, thinks back wistfully on her years in London where she was free from care, pain, or loss. She begins to think that it would be good to escape to London, and leave behind all the grief she's experienced in Milton.