It didn’t violate English law because under English law, it is up to the victim whether or not to press charges against his or her assailants, he declined this and decided to arbitrate the matter with a Sharia court.
It’s no different to a neighbour who declines to press charges against a kid he caught stealing from his house, in favour of an arrangement wereby he is punished by having to perform chores for the neighbour for a month or something along those lines....
That's not entirely true, the police can offer the decision to the victim at their discretion but they can still prosecute, without the co-operation of the victim, if they consider there is a public safety issue, which, in case of their being a knife wielding maniac who is prepared to seriously wound or kill someone, I would say there is a good case.
How, otherwise, would you prosecute a murder? Murder victims NEVER press charges.