Well ... it's a little more complicated than that.
Slavery was abolished for the first time in England in 1102, though it kept coming back; it was abolished again in Cartwright's case of 1569, for instance. However, when people refer to "abolition of slavery" today, they usually mean the abolition of the African slave trade and then of slavery in the New World, notably the Caribbean colonies and the United States.
In Great Britain, many Enlightenment philosophers including Locke and Mill were significant opponents of slavery and the slave trade — although so too were many religious dissenters, notably Quakers, who tended to be better organized and more committed. The mainstream Church of England, in contrast, held many slaves itself; so this wasn't a case of religion vs. irreligion. For that matter, religious toleration, which led to the legalization of the Quakers and other dissenting churches, was itself arguably an Enlightenment project. The Enlightenment was never an expressly atheistic movement in Britain or America; English Freemasonry, rather deeply involved with the Enlightenment, to this day does not accept atheists.
Meanwhile in France and the French colonies in the New World, slavery was abolished by the "Enlightenment" (and "rationalist"!) French Revolution, then shortly re-established by Napoleon. That didn't work out so well for Haiti ...
Meanwhile in France and the French colonies in the New World, slavery was abolished by the "Enlightenment" (and "rationalist"!) French Revolution
To add a related data point, the same thing happened in the rest of Latin America. Abolition of slavery was of the first measures of many of the revolutions against the Spanish in the early 19th century, which were heavily inspired by the Enlightenment. Brazil held out a few decades more, because slavery was a lot more integral to its economy.
I wanted to bring attention to two posts from Razib Khan's Discover magazine gene expression blog (some of you may have been readers of the still active original gnxp) on the polemic surrounding Pinker's The Better Angels of Our Nature.
Relative Angels and absolute Demons (and the related But peace does reign! )
I generally agree with some of his arguments, but found this quote especially as summing up some of my own sentiments: