It sounds bizarre to charge someone with "possession of a knife" when he was inside his own home.

I suspect that Mr. Ali's mistake was to assume the police were there to help him, and he made a statement without asking for his lawyer (I forgot whether that would be a solicitor or a barrister in the UK). Telling the police that he had grabbed the knife for self-defense sounds like a statement made in all innocence by a man who believed he had done nothing wrong.

No disrespect intended to the law enforcement officers and prosecuting attorneys who I know frequent this forum; but there are simply far too many cases where the police have taken advantage of an ordinary citizen's naivete to coax him (or her) into making an incriminating or self-incriminating statement.