That strikes me as a fair assessment, though I would add that the necessity of ennobling free labor and creating a 'disciplined' workforce were important as well. Allow me to quote one of my favorite historians (I've been quoting him a lot today!), David Brion Davis.

"The rise of antislavery sentiment in Britain coincided with an urgent domestic problem of labor discipline and labor management. If abolitionists agreed on the necessity of maintaining order and reforming the manners and habits of the working class, they clearly felt some ambivalence, along with many of their countrymen, toward the changes accompanying early industrialization. The immediate problem did not concern an industrial proletariat, but rather an immense rural labor force that had been uprooted, released from traditional restraints and controls, but not yet deprived of the independence of preindustrial village culture....An attack on the African slave trade could absorb some of the traditionalist's anxieties over the physical uprooting and dislocation of labor. The victims of enclosure were at least not marched in chains to the slave ships or shipped abroad to unfeeling masters." (pp. 428-429)

Now, Chasteen is correct, earlier in his book Davis mentions that British citizens had begun to feel that they were living in a "new empire" based on opening markets and exploiting resources in other countries (even if they had to rely on coerced and ill-treated labor) rather than trafficking large numbers of bonded labors into their holdings (Davis, p. 62). And, of course, Davis would certainly agree that there was a strong humanitarian element in the abolition of the slave trade and opposition to slavery as well. But there were a few other factors Chasteen doesn't mention--not that this is a mark against him, but rather, I'm sure, that he wanted to give a reasonably succinct overview of abolition within the constraints of the space he had. :)

Source cited:David Brion Davis, The Problem of Slavery in the Age of Revolution, 1770-1823 (Oxford University Press, 1999). Also, J. R. Oldfield, in Transatlantic Abolitionism in the Age of Revolution: An International History of Anti-slavery, c.1787-1820 (New York: Cambridge University Press, 2013) mentions some other factors in the abolition of the slave trade; for instance, in the aftermath of the Haitian Revolution, some people in Britain feared that importing more slaves would lead to similar uprisings occurring in British holdings and thus moved to stop the trade.

Various authors can (and have) supposed and ascribed a wide variety of economic or other reasons besides humanitarianism and morality to the British ending of the slave trade.

There is very little evidence (that I can see - perhaps other contributors can find more) to support any arguments made in Parliament, or to influence Parliament, other than humanitarian and morale arguments.

It is particularly difficult to see how abolition of the Slave Trade could in any way, shape or form or turn slaves into consumers of British goods. The abolition of the Slave Trade did not free the existing slaves. Who was supposed to consume all these British goods? The inhabitants of Africa (now presumably being enslaved less, as the trade (in British ships) was abolished)?

The Africans were already consuming British goods and paying for them with slaves. If they did not have the ability to sell slaves, how the heck were they supposed to buy and consume more British goods?

No, the Africans were likely to consume fewer British goods and more goods from countries which still allowed the slave trade. (Which may partially explain why the British were keen to stop all countries engaging in the slave trade, but does not give an economic rationale for Britain abolishing the trade.)

Britain banned the slave trade in 1807.

The Committee for the Abolition of the Slave Trade was formed in 1787 by a group of Evangelical English Protestants. This Committee, led by William Wilberforce, amassed evidence (and propaganda) about the evils of the slave trade, and slowly built up support in Parliament.

By the early 1800s there was a block of 35-40 MPs, known as the "Saints", who were opposed to the slave trade. With the Act of Union with Ireland, in 1800, 100 Irish MPs were added to the House of Commons, many of these were against the slave trade, and the Saints were able to horse trade support for their position with most of the rest of the Irish MPs.

The anti-slave-trade movement was fortunate when Lord Grenville became Prime Minister. He was staunchly against the slave trade, and managed to steer the necessary legislation safely through Parliament, giving passionate speeches in the House of Lords against its immorality.

The Abolition of the Slave Trade Act was passed in 1807, which not only banned all British colonies from importing slaves, and all British ships from participating in the slave trade, but which also committed the British government to seek to persuade all other nations to abolish the slave trade as well.

I see very little evidence for any motivations behind the elimination of the slave trade besides moral and humanitarian motives. (Other than some political horse trading, along the lines of "You support us against the slave trade and we will support something you want".)

Wilberforce, having succeeded in a 20 year fight to abolish the slave trade, decided that this was not enough. In 1823, he helped found the Anti-Slavery Society, which now worked to abolish slavery in the British Empire. They accomplished this (with a few exceptions where slavery was not abolished until 1843) ten years later with the passage of the Slavery Abolition Act of 1833, three days before Wilberforce died.