During the conversation between Thomas Clarkson and William Wilberforce it is questioned how a person can remain loyal to a king who shakes hands with an oak tree and see Germany through his telescope. The quote, spoken by Clarkson, is a reference to King George III who, by the contemporary belief of history and scientific research, was known to have suffered from porphyria which was possibly provoked by his use of arsenic.
The scene where William Wilberforce sings "Amazing Grace" at the card house was actually directly sung from Ioan Gruffudd at that moment. In the last several takes, a playback was used, but it is Ioan singing. Director Michael Apted had no idea whether or not Gruffudd could actually sing. Little did he know, Ioan is an accomplished soloist and choir singer. With a little practice, Ioan performed for the first time on set while the cameras were rolling. All much to the surprise of the cast, crew and director.
After the French Revolution, Charles Fox did not side with William Pitt the Younger. Whilst they were both members of the Whig Party, Fox was a prominent member of a pro-revolutionary faction of the Party, as opposed to Pitt's anti-revolutionary stance. This difference in opinion would typically pit them on opposite sides of the dispatch box.