You are correct, Jabin. Correction noted.
I would add that he was greatly influenced by the Reformed.
'John Newton's Letters to William Wilberforce
Both the importance and the difficulties of your situation, superadded to my regard,
entitle you to a double portion of my thoughts and prayers. [1]
"Sir," wrote William Wilberforce in great distress to John Newton on Saturday 2 December 1785, "I wish to have some serious conversation with you." Though he possibly had not seen him for fourteen years, Newton was the one person whom Wilberforce felt he could trust for spiritual advice in his conversion crisis, knowing that anything he said would be held in complete confidence.
Almost two years later, on Sunday 28 October 1787, with an even deeper, proven friendship and mutual regard between them, it was Newton whom Wilberforce had at his side for further serious conversation on the day that he was to enter in his diary, "God Almighty has set before me two great objects, the suppression of the Slave Trade and the Reformation of Manners." [2]'
https://www.johnnewton.org/Articles/105307/The_John_Newton/archive/The_Complete_Works/Correspondence/William_Wilberforce/background.aspxNewton was obviously big R Reformed.
Moreover, many Methodists were Reformed as well. For Wilberforce was taken to hear George Whitfield preach because Methodism at that time was apart of the Anglican Church.
https://www.baptist.org.uk/Articles/398246/Amazing_Grace_William.aspxWilberforce did in fact reject the 39 Articles.
So you are correct, Jabin. Correction noted.