Why did it take so long?
Although there was always resistance from the enslaved people themselves, amongst the British population at first, it was only a few lone voices that spoke against the trade. The Transatlantic Slave Trade was very profitable for Britain. It provided work, wealth and consumer goods.
The early abolitionists were mainly motivated by religious beliefs. Many were Quakers who, as religious dissenters, were seen as on the fringe of society and even as dangerous fanatics, because of their belief in equality. They were banned from public office and had little direct influence.
Other early abolitionists, like James Ramsey and Granville Sharp, had come to see the injustice of enslavement through individual experiences (in Ramsey's case, when he went on board a slave ship as a doctor and in Sharp's case, when he met an injured former slave). They expressed their opposition in different ways - Sharp through legal means and Ramsey through ministry on the plantations and in his writings. It took a while before such campaigners became aware of each other's experiences.
What events contributed to its formation?
The status of people who were enslaved in the West Indies and then travelled to Britain with the people who had enslaved them, raised questions over the legality of slavery in England and whether these people could be removed from the country against their will. The British defeat in the war in North America, in 1783, saw many people return to Britain, often with former slaves, making more people in Britain aware of the issues.
The presence of this community of black people in England helped kick start the abolition movement. They lobbied the newpapers, supported runaways and followed closely legal challenges, such as those mounted by the abolitionist Granville Sharp.
Those committed to abolition also began to correspond and circulate each other's work. For example, the American Quaker, Anthony Benezet, corresponded with Granville Sharp, as well as writing to many influential people. The African abolitionist, Olaudah Equiano also made contact with Sharp when his friend, John Annis, was kidnapped by his former owner. He also made contact with the Quakers as his 1783 letter demonstrates.
External events, such as the Zong case in 1783 (where 133 sick slaves were thrown overboard, their 'owners' then claiming for the loss from their insurers), caused outrage and raised public awareness. It was this event that encouraged Dr Peter Peckard, vice-chancellor of Cambridge, to set the title of the annual essay competition as, 'Is it lawful to enslave the unconsenting?' It was studying for this essay that led Thomas Clarkson to the cause of abolition.
On 17 June 1783, the Quakers presented Parliament with a petition against the Slave Trade signed by 273 Friends. Following this, a small group of Quakers wrote and circulated anti-slavery literature to newspapers and lobbied Members of Parliament and other notables. Around 1785, an association between the Quakers and Granville Sharp developed, following his correspondence with Benezet.
Then, in 1786, when Thomas Clarkson was looking for a printer to publish his essay to a wider audience, Sharpe introduced him to the Quaker printer, James Phillips (a member of the Quaker abolition committee). Following this meeting on 22 May 1787, the Society for the Abolition of the Slave Trade was formed, consisting of 9 Quakers and 3 Evangelist Christians. Together, Phillips and Clarkson hammered out the tactics of the committee that was chaired by Sharp.
Thomas Clarkson became the backbone and researcher for the cause and Clarkson, amongst others, encouraged the Evangelical William Wilberforce to be the movement's voice in Parliament. They were further supported by a number of African campaigners who had set up their own association to fight slavery, the 'Sons of Africa' group - see Campaign section.