"I am he. With whom have I the honor of speaking?" (Though he has a fairly decent idea.) 

"William Wilberforce, sir."

"I would speak to you regarding the end of slavery, William Wilberforce." 

"In England?"

"For a start."

"Then I suspect we have much to discuss. Walk with me?"

"Gladly." Ipaxalon bids farewell to the crowd, thanking several for their conversation by name, and he and William set out for a walk along the Thames.

"First, I must apologize for my unorthodox approach to getting your attention. I hope it has not cost you credulity." 

"I doubt anyone knows what to think of this occurrence, yet, for good or ill. But perhaps you can enlighten me as to your purpose? Were you sent by God?"

"Not to my knowledge, though perhaps He may have had a hand in my arrival here." Ipaxalon relates an abbreviated version of his tale: a war, a wish, a change of scenery. He describes his past as a warrior against Evil, his encounter with the Shotley, his arrival in London, and his subsequent learning about the slave trade while consulting with merchants. 

Curious. But it is heartening that he saw the grave wrongs besetting England so quickly. How does he propose to help? 

Ipaxalon hoped the abolitionists might have ideas, once they know more. He can be a compelling diplomat, but is more used to taking strategic direction than deriving it himself. Eventually, given what he has seen of this world's combat capabilities, he expects to be understood as an independent military force in his own right, one it would be disadvantageous to offend. But it is better and more robust for a people to learn to abhor slavery on its merits than on practical grounds, and in the short term, he suspects his greatest asset to the movement will be the ability to carry supplies and passengers across great distances instantaneously. 

Instantaneously? How great a distance? 

Ipaxalon explains the function and limitations of the teleport spell. (About 1300 miles, presently.) 

This could be quite useful indeed, particularly for bringing before Parliament those who would bear witness against the horrors of slavery. Is he also offering his aid in promoting piety and virtue, and countering the growing corruption and immorality of the British people? 

That does sound like something he is in favor of, though the details matter. What are some examples of this corruption? 

Drinking, blasphemy, obscenity, profanity, prostitution, cruelty to animals...

...Ipaxalon will stick to abolition for the time being. 

Very well. Before they get into the details of English politics, what would Ipaxalon plan to do if left to his own devices? 

Gather information first, about the nature and provenance of the slave trade and its most important components; visit Africa to speak with those peoples at risk of enslavement about ways that he could help them avert it. His plans would likely change after making a thorough study of the problem, but his current best guess would be to formally petition the government to address Parliament and attempt to sway them to immediate action; failing that; to go directly to the King of England and implore him to act; failing that, to warn the King and Parliament of what would by necessity follow: a series of raids upon the main hubs of slave trade in Africa, freeing slaves and offering support in defiance of their oppressors. The raids would continue until the practice of taking slaves ended. 

The army would be sent to stop him, at that point.

They would lose. 

Truly? 

If cannon, shot, and single-ability magical girls are the extent of the combat capabilities they can bring to bear? If they don't have an ability or combination thereof that happens to perfectly counter everything he can do? Yes.

Given even a small amount of support from the local populace, he strongly suspects his main concern in fighting the entire British army would be minimizing casualties. On their side. 

To be clear, he very much hopes it will never come to this and intends to make every effort to ensure it does not. Far better if the matter can be resolved peacefully. 

...in that case, they have quite a lot to discuss. 

To begin with, a summary of the current work — who in Parliament is sympathetic, who is neutral, who opposes reform, and why. These are the key allies they correspond with, in Africa and America and elsewhere. These are the people Ipaxalon should also speak to, who know more of different parts of the work...