Unpossible. Having eradicated gun violence in their society, the UK bids fair to eliminate knife crime as well.
Even if this Victim-of-White-Supremacy-and Climate-ChangeTMdid stumble over a blade that accidentally fell into another person, there is no way that such an Evil ObjectTM could have found its way to wreak its havoc in such a high-security Safe SpaceTM as one of His Majesty's Prisons.