Dickens was staunchly anti-slavery
When Dickens visited America in 1842, he was horrified by what he saw and heard of slavery in the southern states, and strongly argued for its end. Although this is not now considered a progressive opinion, many Brits at the time still supported or were indifferent to slavery, very unlike Dickens.
Slavery had been abolished in Britain and its colonies through a series of acts passed in 1807, 1811 and 1833, but was still legal and practised in America. Although Britain had a strong abolitionist movement by the time Dickens was writing, the British public were still largely unconcerned with American slavery and did not know its realities.
Although slavery had been abolished in Britain and the British Empire through the 1807 Slave Trade Act, it still existed in America, which had broken away from British rule before the act was passed. Many Brits were indifferent towards American slavery or did not view it as their problem, despite it being caused by British colonial legislation, even though there was by this point a strong abolitionist movement in Britain. When Dickens visited America in 1842, he described in his travelogue "American Notes" how horrified he was by slavery and the American public opinion that supported: "Why, public opinion in the slave States is slavery, is it not? Public opinion, in the slave States, has delivered the slaves over, to the gentle mercies of their masters. Public opinion has made the laws, and denied the slaves legislative protection. Public opinion has knotted the lash, heated the branding-iron, loaded the rifle, and shielded the murderer."  This vehement argument against slavery was not necessarily common at the time, and certainly not common in America, so Dickens' commitment to emancipation must be lauded.
Britain had a strong abolitionist movement by this point and although he had the distinction of being widely published, Dickens was not unique for speaking out against slavery, which the British government had shown it did not endorse. Additionally, it was easy for him to detachedly criticise American slavery as a visitor to America, meaning he did not interrogate the British colonial laws which had made slavery legal and widespread in America, unlike other abolitionists, so although it is good he was openly against slavery, he was not necessarily progressive for it. Additionally, Dickens was not immune from casual racism: in the same section of "American Notes", he includes a racist anecdote about an African-American coachman, calling him "a kind of insane imitation of an English coachman". Later in his life, in 1868, he argued against allowing African-Americans to vote.
Many Brits were not concerned about slavery in America because it had been abolished in Britain, meaning they did not think about it. Dickens used his platform to discuss emancipation. Dickens was horrified by American slavery when he travelled to America in 1842 and used his platform to write a strong argument against it.
Rejecting the premises
People, including Dickens, were unwilling to recognise that slavery in America was the direct result of British colonial rule. In the same argument against slavery, Dickens includes a racist anecdote about an African-American coachman. Being anti-slavery was not the same as being anti-racist.