The old Usury Laws were repealed in 1854, and for the rest of the Victorian period, up until the 1901 Moneylending Act, money lending was largely unregulated.
Most household bills were paid quarterly or annually, so it was easy to live beyond your means. Creditors could pass an unpaid bill to a moneylender, who could then charge interest.
As a young man Trollope himself had dealings with a moneylender. He tells the story of a tailor’s bill for £12 which was passed to a moneylender, who loaned the young Trollope a further £4. In time this debt ‘grew monstrously’ so that Trollope eventually came to owe £200. In The Three Clerks, he writes of a similar situation and used this experience to base the character of his many of his moneylenders on.