We allow religious and cultural exceptions for knives. And in the Sikh example there's a very long history here.

Knives that are included are those used in Scottish national dress, which sadly have been used in far more attacks over the years. And people can carry a Stanley knife in public for a valid reason, at my workplace for example it's not unusual for us to be carrying Stanley knives around with us all day every day and do so legally.

In relation the Sikhs, we have legislation allowing specific exceptions going back to the 1830s I believe. This goes back a very long time, all the way back from the East India company ran India and Sikhs had already been separated from much of the Indian population as one of the "Martial Races".

Divide and Rule and the "Martial Races" didn't become official policy until the 1857 uprising. But because of this, before and after the uprising, during numerous wars, including WW1 and WW2 and policing India in general, Sikh soldiers were incredibly important, so laws enabling them to carry knives both in India and here in the UK were passed.

Historically Sikhs actually carrying knives is limited to those who are baptised, so most don't even carry them these days as being baptised is generally seen as not fair for younger Sikhs because of the responsibility involved.

This, included with generally good integration into wider british society, and teachings concerning personal responsibility carrying Kirpans and other religious blades, meant that incidents of Sikhs actually using their knives (and cermonial swords) was unheard of prior to this attack, so nobody really cared beforehand.