Just to be clear, there is no specific exemption for kirpans. The exemption is for knives worn for religious reasons or as part of a national costume, hence why Scotsmen wearing traditional Highland dress can have a dagger (a sgian-dubh) in their stocking. Note that most Sikhs wear a small knife hidden from view. Some are members of an order called the Nihang who traditionally wear a second knife that is fully visible. Digwa, Nowak's killer, observed that tradition, but his father and brother did not.
The amount of knife crime committed by Sikhs is vanishingly small. Knife crime by any ethnic group using knives worn for religious purposes or as part of a national costume is pretty much non-existent, which is, perhaps, part of the reason we have heard so much about this case. The vast majority of murders in the UK get little or no coverage in the national press.
We will never know if banning knives would have made any difference in this case. Given Digwa's obsession with weapons, he may have carried a knife anyway, even if it was illegal. Almost all knife crime is committed using hidden knives being carried illegally. Maybe we should stop knives being worn for religious reasons or as part of national costumes, but I would be wary of a knee jerk reaction to this particular case.
Also, I'm not sure why OP is comparing what she is allowed to take onto a flight with what people are allowed to carry in public generally. There is no restriction on the amount of liquid you can carry in public. Indeed, at Heathrow, Gatwick, Birmingham, Bristol and Edinburgh airports you can board a flight with a container carrying up to 2 litres of liquid. You are not allowed to take any knife onto a flight at any airport for any reason.