This is incorrect, the knife he used to stab Henry was not unlawful. The relevant part of the judgement is here:
10. You were sober but were carrying a large Sikh dagger in a sheath attached to a belt over the outside of your clothing. It is a strict requirement of the Sikh faith to have a knife, called a kirpan, at all times. Generally, this will be a small knife, hidden from view, often on a length of cord and worn around the neck. You had that but, in addition, the large dagger in a sheath. You are a member of an order of Sikhs called the Nihang who have a tradition of having a second knife, or kirpan and that is often fully visible, believing that the guru will look favourably on that. You observed that tradition in your everyday life, at work and in public. However, it was not a strict requirement; that is borne out by the fact that neither your brother nor father who arrived on the scene after you had stabbed Henry were so dressed. According to Professor Gurnam Singh, Professor of Sociology and an expert in the field: “Over the last 30 years, there has been a trend towards younger people wearing a kirpan with pride, in a desire to express their cultural identity. They see it as an act of resistance to being denied the ability otherwise to display their identity.”
11. The privilege extended to practising Sikhs of being allowed to be in public with a bladed article and, particularly in respect of the large dagger, a highly dangerous weapon, easily accessible to the wearer, brings with it huge responsibility. 12. It is a fundamental principle of Sikhism that any kirpan is worn as a symbol of religious faith and is never to be carried for an offensive purpose. The legal approach to the carrying of such a knife, as long as the blade length does not exceed 9 inches, is that an offence of Having a Bladed Article in a public place will not be prosecuted; there has been an acceptance that its possession in those circumstances, can amount to a good religious and, therefore, legal reason for having it. The blade of the knife will not be on display; either it is under clothing or, alternatively, in a sheath. For both, it is a religious and, consequently, legal requirement that a kirpan should only be used offensively as a last resort, which would include its use in legal self-defence. In other words, only if use is necessary and, if so, reasonable in the circumstances. It is obvious that for use to be reasonable, any perceived threat justifying its use would only be in circumstances of great seriousness and urgency.
Sorry it's long, but I have repeatedly linked to the judge's sentencing remarks and for some reason you seem not to want to read them.
The full judgement is here:
Digwa Final Sentencing Remarks
It's only 6 (short) pages long- it will literally take you 5 minutes to read.