What compelled John Wesley to take up the cause of Abolitionism? In 1778, John Wesley published his Thoughts upon Slavery, where he wrote:

The variety of ways that slavery appears make it almost impossible to convey a just notion of it ... First in what manner are they procured? Part of them by fraud ... But far more have been procured by force. The Christians landing upon their coasts, seized as many of them as they could find - men, women and children - and transported them to America ... It was some time before the Europeans found a more compendious way of procuring them: by prevailing upon them to make war in each other and sell their prisoners. Till then they seldom had any wars, all was peace and quiet. But the white man taught them drunkeness and avarice and hired them to sell one another. Nay, by this means even their kings are induced to sell their subjects ... As for the punishments that are inflicted upon them, say Sir Hans Sloan, some are gelded, others have half a foot chopped off ... others cut off an ear, and constrain them to broil and eat them ...
How can Britons can so readily admit a change in their disposition and sentiments as to practise in America what they abhorred and detested in Britain can be accounted for no other principle than of being the natural effect of slave-keeping which as the celebrated Montesquieu observes 'insensibly accustoms those who are in the practise of it, to want all moral virtues, to become haughty, hasty, hard-hearted, passionate, voluptuous and cruel.'

More, forty years earlier when he had been in the United States with his brother on missionary work in 1737-38 he recorded in his journal the barbaric treatment that:

daily practise upon their fellow creatures [inclufing whipping, driving nails through their ears and drawing teeth] ... even giving a white child a slave of his own to tyrannise ... [describing them] as shocking instances of diabolical cruelty

Given the interest that Britain had in maintaining the slave trade it's not suprising to discover that whenever Wesley preached against the slavery trade he put his life at considerable risk. For example, when he preached in Bristol, one of the foremost slave-trading ports in Britain, a disturbance broke out.
Q. Given this, what compelled Wesley to take on slavery and become one of the early adopters of abolition?
Moreover, given the shocking scenes of 'diabolical cruelty' he saw in the then slave owning United States why did it take so long to take up the cause?
Finally, how significant was his preaching in the context of the abolitionist movement - my own understanding - from being told this at school - was that Wilberforce was the main force behind it
You ask what compelled Wesley to take on slavery and become one of the early adopters of abolition.  I found a somewhat lengthy and wordy paper on this subject and have extracted a few paragraphs that may be helpful:  https://brycchancarey.com/Carey_BJRL_2003.pdf

2/16: Wesley's life coincided with the height of the British transatlantic slave trade. Britain had been a minor participant in the trade since the late sixteenth century. In 1660, Charles II had given the trade systematic government support and British involvement grew at a sharp rate over the following years. With the treaty of Utrecht of 1713, the event which concluded the War of the Spanish Succession Britain won the right to supply the lucrative markets of Spanish America with slaves. From that point onwards, Britain was by far the most significant participant in the international slave trade, responsible for transporting thousands of abducted Africans across the Atlantic every year, both to Spanish America and to British colonies in the Caribbean and North America. From the start, there had been those who opposed slavery and the slave trade, but their voices were rarely heard until late in the eighteenth century. In the decade following American independence, a public pressure campaign to outlaw the slave trade to the remaining British colonies attracted widespread support from the British public, who made their views known through local meetings, a mass petitioning campaign and a consumer boycott. Hundreds of publications opposing both slavery and the slave trade were printed and distributed between 1785 and 1795. The campaign was not immediately successful but, following a ten-year lull, the British slave trade was abolished by law in 1807. After another vigorous campaign in the 1820s and 1830s, the abolition of slavery in British colonies followed, with the emancipation of slaves taking place by 1838.


4/16: Wesley and the Abolition Movement: Although Wesley claimed to have been opposed to slavery from the first time he heard of it, we have no way of knowing if this is true. Neither can we know with certainty when he first heard of slavery. He might have come into contact with slaves in England. In the early eighteenth century, it was fashionable for aristocratic women to employ black pageboys, and young liveried slaves were not an uncommon sight. Slaves were also brought to England by visiting colonists and by officers in both the Royal Navy and the merchant marine. However, if Wesley had encountered slaves such as these, he does not mention it in his letters or Journals. Without doubt, he did come into contact with slaves during his period in North America (1736-37).


13/16: Extract from Wesley’s Journal, August 12, 1772: “Wed. 12. - In returning I read a very different book, published by an honest Quaker, on that execrable sum of all villainies, commonly called the Slave-trade. I read of nothing like it in the heathen world, whether ancient or modern; and it infinitely exceeds, in every instance of barbarity, whatever Christian slaves suffer in Mahometan countries.”  William Wilberforce too, an evangelical rather than a Methodist, made it a constant theme in his writings and speeches, and even went as far as to write a book called A practical view of the prevailing religious system of professed Christians in the higher and middle classes of this country contrasted with real Christianity, which appeared in 1797.


16/16: In Thoughts upon slavery: Sharp and Benezet's work was legalistic, rational, primitivist and pious, but it was seldom sentimental... Wesley's arguments against slavery were not often original, but his major innovation was to introduce a sustained and concentrated sentimental rhetoric into the antislavery debate. Most of the many campaigners against slavery in the coming decades would, wittingly or otherwise, follow his example.

You also ask how significant was Wesley’s preaching in the context of the abolitionist movement.  Brothers John and Charles Wesley had a huge impact on Christian revival in Britain, especially in the north of England.  They constantly travelled from the east to the west coast preaching and establishing new churches wherever they went.  Given the vast number of sermons delivered by John Wesley, it would be difficult to pin down the significance and the influence his sermons on slavery had.  Unfortunately, I have to sign off for tonight but I hope to return to this question tomorrow.  I will try to find a suitable link/article about his sermons on slavery. To be continued...
John Wesley was ordained as a deacon in 1725 and as a priest in 1728.  During the early years of his ministry many of his sermons were unpublished.  However, his preaching had a huge impact.  Here are a few extracts from an article about him:

What kind of a preacher was John Wesley? To understand the impact of his preaching we must first appreciate his commitment to the task. He travelled some 225,000 miles throughout Britain and Ireland on horseback. At a conservative estimate he preached more than 4,000 sermons, some to congregations in excess of 20,000. He would regularly preach four or five times a day. His sermon register from January 1747 to December 1761 reveals that during this period he preached 7,000 sermons on 1,354 texts. He would regularly preach at five in the morning. Indeed, he laid great store by it, seeing it as 'the glory of the Methodists'.


John Wesley preached what he knew a particular congregation needed to hear. Thus, whilst some sermons had as their aim the winning of souls for Christ, others were directed at those who were already believers and concentrated on incentives to holy living. His conviction that all life belonged to God meant that no subject was beyond the scope of his preaching; and he was not averse to speaking about the use of money, the use of leisure, national sins and miseries as well as the doctrines at the heart of the Christian faith. But the decision about what subject to preach to which congregation was determined solely by the spiritual condition of his hearers.
https://www.methodistevangelicals.org.uk/Articles/523424/John_Wesley_Preacher.aspx

In 1739 John Wesley took up George Whitfield’s invitation to take over the work he had begun in Bristol and started to preach out in the open air.  It has been calculated that of the 500 sermons delivered between April and December 1739, only eight were in churches.  In this way, Wesley was able to communicate with a much larger audience.  The article in the link above says nothing about sermons on slavery, although he undoubtedly delivered sermons on this topic, especially given his association with George Whitfield and Bristol, a port infamous for its participation in the slave trade.
I regret to say I have been unable to find any of Wesley’s published sermons that deal with slavery and the abolition movement.  I found an article on the 44 sermons contained in ‘Sermons on Several Occasions’ (published in four volumes between 1746 and 1760) but it does not mention slavery: https://www.wesley.cam.ac.uk/wp-content/uploads/2015/05/10-wesley.pdf
Nonetheless, it is clear that John Wesley reached thousands of ordinary people as well as some of the great political “movers and shakers” in his day.  As the above article concludes “The heart of the matter lies not in what Wesley did, but what God did through him.”
With regard to William Wilberforce (1759-1833), who was an English politician and Member of Parliament, he was a key figure in ending the abominable slave trade that flourished at that time. His campaign was eventually successful, and he is lauded today for his courage and commitment to Christian principles.  He became a Christian in 1785 and was himself influenced by other Christians:

In 1787, he came into contact with Thomas Clarkson and a group of anti-slave-trade activists, including Granville Sharp, Hannah More and Charles Middleton. They persuaded Wilberforce to take on the cause of abolition, and he soon became one of the leading English abolitionists. He headed the parliamentary campaign against the British slave trade for twenty years until the passage of the Slave Trade Act of 1807...  His campaign led to the Slavery Abolition Act 1833, which abolished slavery in most of the British Empire. Wilberforce died just three days after hearing that the passage of the Act through Parliament was assured.  https://en.wikipedia.org/wiki/William_Wilberforce

Wilberforce was also influenced by other abolitionists such as John Newton (1725-1807) and George Whitfield (1714-1770):

John Newton (1725—1807) was a Christian, abolitionist, and hymn writer best known for his song “Amazing Grace.” John Newton’s work for the abolition of slavery in England, propelled by his faith, greatly influenced others, including Parliament member William Wilberforce. Wilberforce worked tirelessly to end slavery in the British Empire, and in 1787 Newton wrote Thoughts upon the African Slave Trade to aid Wilberforce’s cause. John Newton lived to see the passing of the Slave Trade Act of 1807, which made slave trading illegal. Slavery was finally abolished in most parts of the empire in 1833. https://www.gotquestions.org/John-Newton.html


George Whitfield (1714–1770) was converted to Christ while a student at Oxford University. There he became friends with John and Charles Wesley.  [Whitfield] made seven trips to the American Colonies and influenced a young William Wilberforce.  https://www.gotquestions.org/George-Whitefield.html