Frederick Douglass was quite clear on the reason for his trip to England.  In his famous speech in Leeds, England, on December 23, 1846, Douglass stated:

"I have come here to call your attention to America, where there are three million of persons in slavery -- three million of human beings liable to be put on the auctioneers block and sold as beasts and swine -- and this in a nation which declares that all men are equal."

With the Emancipation Proclamation still seventeen years in the future, and the country increasingly divided over the issue of slavery, Douglass believed that the Blacks' only recourse in their struggle for freedom lay in the country from which the United States of America sprang:  England.  The plight of slaves was sufficiently dire and the American South had institutionalized slavery so deeply into its economy and culture that Douglass concluded that foreign attention was the only answer.  Once again, from his speech in England:

"There are, however, persons who honestly believe that we should not interfere with America in this matter.  This objection would be fairly put forth, if we called for political interference, or for the interference of your arms.  But we have no such measures to propose to you.  We ask you to interfere by way of correcting the moral sentiment of America.  This you have a right to do; you have a right to speak."

Douglass understood that the United States, while having successfully fought for independence from the Crown, including reaffirming that independence in the War of 1812, was still a part of a community of nations, and that it could not long exist in a moral or political vacuum -- that it was, in fact, susceptible to foreign condemnation.