From Chapter 79:

Harry nodded, his mouth set. "Exactly what sort of penalty is Hermione facing? Snapped wand and expulsion -"

"No," Severus said. "Nothing that light. Are you willfully misunderstanding, Potter? She is facing the Wizengamot. There is no set penalty. There is only the vote."

Harry Potter murmured, "The rule of law, in complex times, has proved itself deficient; we much prefer the rule of men, it's vastly more efficient... There's no constraining legal rules at all, then?"

Light glinted off the old wizard's half-moon glasses; he spoke carefully, and not without anger. "Legally, Harry, we are dealing with a blood debt from Hermione Granger to the House of Malfoy. The Lord of Malfoy proposes a repayment of that debt, and then the Wizengamot votes on his proposal. That is all."

I read this as implying that Magical Britain doesn't actually have a codified legal system. (Although it seems like a waste of the Wizengamot's time to vote on all crimes; perhaps there's some commitee that the delegate minor crimes to?).