Pentonville, the suburb where Mr. Brownlow lives, wasn’t a random choice on Dickens’s part. George Cruikshank, the illustrator of Oliver Twist and a buddy of Dickens’s, lived there.
William Makepeace Thackeray, another famous Victorian novelist, thought that all Newgate novels were dangerous and immoral, and could incite working class readers to crime. In fact, there actually was a famous case of a servant, named Courvoisier, murdering his master and claiming that he was inspired to do so by reading Jack Sheppard, a famous Newgate novel by Dickens’s buddy, William Harrison Ainsworth. Thackeray was so disgusted by the whole thing that he wrote this essay after going to see Courvoisier’s public execution. The lines criticizing the "immorality" of Oliver Twist are particularly famous. (Source)
Bill Sikes was based on a real, historical criminal named "James Sikes," a.k.a. "Hell and Fury." James Sikes was a companion of Jack Sheppard, the criminal protagonist of William Harrison Ainsworth’s historical novel that came out at the same time as Oliver Twist. Ainsworth and Dickens were friends, so it’s easy to assume that Ainsworth, who was researching the historical James Sikes for his own novel, gave Dickens the idea to use the name for his fictional criminal in Oliver Twist. (Source)