You can carry a knife with a reasonable sized blade (under 75 mm) as long as you can justify why you have it.
You are unlikely to be stopped and searched unless you are involved in an assault or fight.
If you are Camping or Fishing or even using the Knife to peel your Fruit you would be ok.
Leave any knife in your checked in Luggage when flying obviously.
The laws here appear strict but the reality is common sense is applied unless you are a troublemaker.