Maybe I'm missing something; but it looks to me like standard procedure.
Police arrive and finda dead body. Who killed it? That guy with the knife wounds probably. Oh, hesays it was self-defense. OK, lets get hold of him until we find outfor sure--lots of murderers have claimed self-defense.
Not that I like that scenario.
I'm just not convinced Mr. Singh is in any more than usual trouble. I don't know English law, but I think there won't be any charges.