September - November 1872: Twain's plan ..."was to travel through various parts of the British Isles to collect material for a book that would do for that country what Innocents Abroad had done for Europe and the Holy Land. A second objective was to secure a British copyright for Roughing It."
"He arrived in England in early September, and from that time until November 12, when he returned home, he was so frequently entertained by the literary and civic leaders of London that he scarcely had time for anything else."
June - October 1873: "By early June the Clemenses were in London, comfortably housed in the Langham Hotel, Portland Place and Regent Street, where their apartment soon became a gathering place for London's literary notables, During July and August they toured Scotland and Ireland. Then, in September, after two weeks of busy sightseeing and shopping in Paris, they returned again to their rooms at the Langham." (Lorch pg 137)
October, 1873 - January 1874: "After only six days in the United States Mark Twain returned alone to London, arriving there about November 20, and again took up residence at the Langham." (Lorch pg 143).