One factor could be the role of the media in Gandhi's portrayal. I recently heard an interesting talk on this, were his influence on British and, to a smaller degree, North American newspapers was underlined - keep in mind that Gandhi was a prolific writer and editor of newspapers himself, and was clearly aware of the great media interest in his person, especially after the salt marsh. A few journalists received exclusive access to contuct interviews (apparently he formed a friendship with William Shirer), which he then looked over before publication.
Another issue were attempts to influence a North American intervention in the independance struggle on the Congress Party's behalf (possibly then influencing Britain in this way), also in part through its positive description in American media. Although these attempts were not succesful in the long run, it seems quite possible to me that public opinion in Britain itself would have been influenced, also by Gandhi's humble appearence and clothing seen in the photos. Again, as he had studied in England he surely had a good grasp on social conduct there. I could also imagine that there was support for Indian independance as a movement far away from England, whithout having impact on existing racial tensions inside the country, and that members of the English working class were sympathetic to lower classes' struggles in India, personified amongst others by Gandhi.
I hope this helps, and that others can contribute more insights and factors.
Edit: The talk was given by Dr. Chandrika Kaul, whose book covering also this topic, 'Communications, Media and the Imperial Experience. Britain and India in the Twentieth Century' seems to have been published.