A capital is a "capital city": the city where government is centered. "London was the capital of enormities and of proud rebellion" means something like this: "London is a capital city that is famous because everything there is very big: the statues, the churches, the bridges. Even the egos and patriotism of the people who live in London are big, which explains why they rebel whenever someone tries to control them or make new laws that limit them, or when enemies try to take over London. The people want to feel big and important and powerful, like everything else in London; and other people trying to tell them how to live makes them feel small."