This is a great question because Dickens had personal issue with some of the child labor issues going on in England during that time, this is why he wrote Oliver Twist.
During that age many children were used and keeping them uneducated meant they could be used in factories. Laws didn't protect children. Mrs. Joe wants to profit from Pip, and if she prepares him to be educated, he cannot work in the forge and bring money home for her. It is an attempt to control Pip as long as possible.
In this passage, Mrs. Joe is being compared to the government of England. By doing this, Dickens is saying that the English government does not want its lower classes to be educated. Dickens is saying that the government fears that an educated lower class will be more likely to rise up and rebel against the government.
In this passage, Joe (and to some extent Pip) represents the lower classes. They are to be kept ignorant so they will not want to rebel and so that if they do want to rebel they will be too ignorant to do so.