What's the main point in chapter 20 for the book Chains?
I would argue that there are two important sections to this chapter. The first is when Isabel goes to the market to do the shopping for the family and sees the destruction of a statue of King George III. This is obviously symbolic of the decline of the strength of the British army, and this is supported by the fact that the statue is made out of lead covered in gold leaf. Its appearance is so much more impressive than the reality, and this is important in the way that the forces of Britain are portrayed.
Secondly, when Isabel returns home from the same trip, she encounters a very different kind of home because Madam visited the wife of the preacher. The preacher's wife gave her a lecture about looking after her slaves and servants better, and so when she returned she baked gingerbread for her slaves and servants and gave them a jar of sweet milk to drink.