Was William Wilberforce a Quaker?

No, Wilberforce was not a Quaker, he was an Evangelical Christian, and member of the Church of England. At the time of the battle against slavery Wilberforce also was involved with reformation issues. He appears to have been more concerned with Humanitarianism, and believed the influence of the church was necessary to achieve this end. That was a view he shared with Quakers like those pesky Americans.